LAST UPDATED Sep 13th, 2018 [Added some new 24/7 streams]
There is a TON of full adult swim shows that are readily available to steam on their website or elsewhere, and it can be crazy trying to find it all. I've attempted to compile a comprehensive guide for every adult swim original that can or cannot be streamed.
EDIT: So this was featured on adult swims front page, which was cool... until Turner came in and took down most of the youtube videos that weren't on adult swim's channel. It sucks because it was mostly the pilots that you can't find virtually anywhere. I've marked through any video thats been taken down.

Entire Series Free on Website and App

12 oz. Mouse
Apollo Gauntlet
Assy McGee
Ballmastrz: 9009
Black Dynamite
Check It Out! with Dr. Steve Brule
Decker
Delocated
Fat Guy Stuck In Internet
FLCL
Home Movies
Hot Streets
Joe Pera Talks With You (includes the infomercial and special)
King Star King
Loiter Squad
Lucy, The Daughter of the Devil
Metalocalypse (includes the rock opera)
Million Dollar Extreme Presents: World Peace (Ep 6 Uncensored Extended)
Minoriteam
Mongo Wrestling Alliance
Mostly 4 Millennials
Off The Air
Perfect Hair Forever
Samurai Jack (includes the original CN seasons!)
Sand Whale and Me
Saul of the Mole Men
Space Dandy
Space Ghost Coast To Coast
Squidbillies
Stroker and Hoop
Superjail!
Tender Touches
The Brak Show (War Next Door is just a 10 second clip, and since I know you're reading this adult swim please fix this)
The Drinky Crow Show
The Heart, She Holler
The Jellies
The Restless Bell
The Rising Son
Titan Maximum
Tom Goes To The Mayor
Xavier: Renegade Angel
You're Whole

Entire Series Free on Other Sites

Soul Quest Overdrive (Pilot, 1, 2, 3, 4)

Almost Entire Series

These shows have nearly every episode on the site but are missing a few. I'll provide links to the missing episodes if available.
Aqua Teen Hunger Force - Full seasons 1-9, partial seasons 10 and 11 which are available on Hulu
Dream Corp LLC - Missing "You Down with OCD?" but this changes from time to time. There's usually at least 5 episodes. Available on Hulu.
Mary Shelley's Frankenhole - Missing "Mother To Be-sa", an unaired banned episode. (Download)

Infomercials

All infomercials (specials that premiered at 4am) are on the website, but they're split up. Some are listed as infomercials, some as specials. Sorted them all here in chronological order. Youtube links included if available.
Icelandic UltraBlue (Youtube)
Aziz Ansari's Dangerously Delicious (thanks metaldanirl)
Swords, Knives, Very Sharp Objects and Cutlery (Youtube)
For-Profit Online University (Youtube)
Live Forever As You Are Now With Alan Resnick (Youtube)
Broomshakala (Youtube)
Fartcopter (Youtube)
The Salad Mixxxer (Youtube)
Alpha Chow (Youtube)
Goth Fitness (Youtube)
Too Many Cooks (Youtube)
The Newbridge Tourism Board Presents: "We're Newbridge, We're Comin' To Get Ya!" (Youtube)
In Search Of Miracle Man (Youtube)
Smart Pipe (Youtube)
Book of Christ (Youtube)
Frank Pierre Presents: Pierre Resort Casino (Youtube)
Unedited Footage of a Bear (Youtube)
This House Has People In It (Youtube)
Joe Pera Talks You To Sleep (Youtube)
Big Grams: "Born to Shine" / "Run For Your Life"
M.O.P.Z. (Youtube / Slowed Down Version)
Giles Vanderhoot (Youtube)
Live at the Necropolis: Lords of Synth (Youtube)
NewsHits (Youtube)
Mulchtown (Youtube)
Check It Out! With Scott Clam
The Suplex Duplex Complex
Wet Shapes
Innovation Makers: The Coyote Suit
A Message From The Future
Cool Dad - Official Trailer
Dayworld
Final Deployment 4: Queen Battle Walkthrough (Youtube)
Food: The Source of Life

Specials

Adult Swim Golf Classic (Extended) - Adam Scott and Jon Daly play as golfers who share their names
Anime Talk Show / Adult Swim Brain Trust - In 2004, Perfect Hair Forever aired its pilot instead of the announced Squidbillies pilot (Here are the bumps "explaining" it: 1, 2, 3). This special hosted by Space Ghost interviewing Meatwad, Sharko, and Early followed it.
Art Prison - A pilot just turned into a one-off special
Bagboy - Check It Out! spin-off
Brett Gelman's Dinner in America - Includes all three specials
Bushworld Adventures - 2018's second April Fools' prank. It's Australian Rick and Morty!
Cool 3D World "Spurt" - Was once removed from Youtube but now it's back
Earth Ghost - Part of 2011's April Fools' prank, aired after The Room.
First Annual Stupid Morning Bullshit Invitational On The Green - A golf match between Mike Lazzo & Andrew Choe
FLCL: Alternative (Subbed) - Episode 1, part of 2018's first April Fools' prank.
Freaknik: The Musical - Hour-long special starring T-Pain that spawned from That Crook'd 'Sipp
The Greatest Event In Television History - Includes all four specials. Created by Adam and Naomi Scott
Mother, May I Dance with Mary Jane's Fist?: A Lifetone Original Movie - Based on a play of the same name by Mary Elizabeth Ellis and Artemis Pebdani of It's Always Sunny in Philadelphia fame
Mr. Neighbor's House - Includes both specials. Parody of Mr. Roger's Neighborhood
Ranger Smith - Boo Boo Runs Wild - Two shorts created by John K. that aired on Cartoon Network before
Scavengers - Toonami special
Soft Focus with Jena Friedman - I don't have much of a description for this, but it's good.
The Xtacles (1,2) - Frisky Dingo spin-off
Toonami Countdown - Supercut of Toonami's 2017 immersion event
Young Person's Guide to History - Includes both parts

Pilots

These aren't all of them, just the ones that are relatively easily available. Turner keeps taking down the Youtube links, i'll update when I can.
Bad Guys
Captain Sturdy: The Originals
Captain Sturdy: Back In Action
Cheyenne Cinnamon and the Fantabulous Unicorn of Sugar Town Candy Fudge
Chuck Deuce
Coffin Dodgers
Duckworth
Finkel Files
Filthy Sexy Teen$
Gigglefudge U.S.A.
Hindenburg Explodes!
Korgoth of Barbaria
Let's Do This!
Let's Fish (Youtube / Vimeo)
Lewis Lectures
Lowe Country (pt. 2)
Neon Knome
Penguins Behind Bars (Youtube)
Saddle Rash (Youtube)
Snake n' Bacon
Southies
Spacecataz
Stiff
That Crook'd 'Sipp (Backup)
The Best of Totally 4 Teens (Youtube)
The Groovenians
The Mark Lembeck Technique
The New Big Ball With Neil Hamburger
The Pound Hole
The Shivering Truth
Tigtone and the Pilot
Trap Universe
Tropical Cop Tales
Übermansion
Welcome to Eltingville (Vimeo)
Yappy Broads

Online Series

I'm only going to list the full series (Thanks bnjrivera for some of these)
Adult Swim Celebrity Poker Tournament Royale
Carl's Stone Cold Lock of the Century of the Week
Daytime Fighting League (Season 1 on Youtube)
Electronic Game Information
On Cinema
Ricking Morty
The Cry of Mann: A Trool Day Holiday Spectacular In Eight Parts
The Tim Heidecker Trial
Yoga Bro

24/7 Commercial Free Marathons

Dream Corp LLC
Off The Air
Rick and Morty
Robot Chicken
Superjail!
The Eric Andre Show
The Venture Bros.
Williams Stream (Variety of random originals) - The format seems to be 1-2 episodes per show, and the selected episodes usually stay for weeks
App Only:
Aqua Teen Hunger Force
Black Jesus
Metalocalypse
Mr. Pickles
Squidbillies

The Rest Are Not Complete And/Or Free

Full series with Hulu subscription (Free partial series on adultswim.com):
Black Jesus (as)
Brad Neely's Harg Nallin' Sclopio Peepio (as)
China, IL (as)
Dream Corp LLC (as)
Eagleheart (as)
Frisky Dingo (as)
Harvey Bridman: Attorney At Law (as)
Hot Package (as)
Moral Orel (as)
Neon Joe: Werewolf Hunter (as)
NTSF:SD:SUV:: (as) - Complete Season 3 on internet archive
Rick and Morty (as)
Sealab 2021 (as)
Tim and Eric Awesome Show, Great Job! (as) - 10 Year Anniversary Special only on [as]
Tim & Eric's Bedtime Stories (as)
The Boondocks - No free episodes on [as], first and second seasons on internet archive
The Eric Andre Show (as) - Eric Andre Does Paris only on [as]
Your Pretty Face Is Going To Hell (as)
Partial with Hulu:
Aqua TV Show Show / Aqua Teen Hunger Force Forever - rest on [as] (see above)
Mr. Pickles (as)- Missing season 3
Robot Chicken (as) - Missing season 9
The Venture Bros. (as) - Season 7 will come and go on [as]
Partial on Website and other sites:
Mike Tyson Mysteries - Few episodes of season 3 on website
Newsreaders - Complete season 1 on internet archive
Childrens Hospital - Seasons 1, 2, and 3 complete on internet archive
​
Not on Website, Lost Media, and Others:
Jack and Triumph Show - rights issue with NBC, no free streaming links atm, on itunes though
​

Bonus Archived Blocks / Non-Originals / Misc.:

I will be adding more to this list as I come across them
April Fools' Prank 2010 - Cut of all the bumps in between The Room with Space Ghost interviewing Tommy Wiseau
April Fool's Prank 2011 - The Room full block with Earth Ghost
April Fools' Prank 2012 - The Return of Toonami full block (Bleach, Dragon Ball Z, Mobile Suit Gundam Wing, Tenchi Muyo! Ryo-Ohki, Outlaw Star, The Big O, Yu Yu Hakusho, Blue Submarine No. 6, Trigun, Astro Boy, and Gigantor)
April Fools' Prank 2018 - Subbed Toonami (FLCL Alternative, Mind Game (full movie), JoJo's Bizarre Adventure: Stardust Crusaders, Hunter x Hunter, Naruto: Shippuden, Space Dandy, Cowboy Bebop, Ghost in the Shell: Stand Alone Complex 2nd Gig, and Scavengers)
Daylights Savings 2017 - Aqua Teen Hunger Force Colon Movie Film For Theaters aired in the extra hour for DST. Includes commercials and bonus episodes of King of the Hill, Rick and Morty, and Ben 10
Aqua Teen Hunger Force Banned Boston Episode - Unaired episode about their Boston bomb scare. Uncensored and unfinished!
Clerks: The Animated Series - aired Friday nights on adult swim in 2008
High School USA! - Fox series made by Dino Stamatopoulos (Moral Orel, Frankenhole). I'm not sure if this aired on adult swim or not.
Mission Hill - Classic old syndicated show. Individual episode playlist on internet archive or Youtube.
The PJ's - Old Fox series by Eddie Murphy.
Wonder Showzen - NOT an adult swim show but for anyone who is a fan of PFFR (Xavier, The Heart She Holler), this is an essential watch. Besides, adult swim wanted this show anyway.
If you notice any broken or missing links, please let me know! Likewise, if you have any material that could go here, leave a suggestion (I've personally been specifically searching for full blocks of their 16th Anniversary weeks). Happy Streaming!