Cameos (Spoilers I guess? lol)

[ุ]

Codes - What it unlocks:

Adultswimgames - Charecter from Super House of Dead Ninjas
Asteroidbase - Lovers in a Dangerous Time Charecter
Breakfall - Starwhal
Capy - Capy Games Devs, Charecters from Critter Crunch and Super Time Force
Cellardoor - Knight from Rogue Legacy
Dennaton - Hotline Miami Character
DenisTalbot - Denis Talbot
Destructoid - Mr.Destructoid
Dom2d - Dom2d, Dev of Mecenary Kings
Doseone - Doseone
Drinkbox - Guacmeelee, and UAY Chivo from Guacamelee!
Egoraptor - Arin Hanson aka Egoraptor
Funhaus - All the members of FUNHAUS on Youtube.
Halfbot - Ash from Bulletage
HUMBLEDUSTHEARTS - Figet from Dust: An Elysian Tale
Ironhide - Vez'nan from Kingdom Rush
Jesse Cox - Jesse Cox of OMFGCATA on Youtube
Juicybeast - The devs of this game!
Justinchan - Justin Chan
Lachhh - Lachhh and Bezerk Studio Avatar
Markiplier - Markiplier on Youtube
Mayhow - Omnomnom, and Marion Esquian
MCB - Marie-Christine Bourdua of Torontaru
Messhof - Fighter from Nidhogg
Oquonie - Neomine from Oquonie and its respective devs
Polytron - Gomez from Fez
Sauropod - Builder from Castle Story
Teammeat - Developers of Super Meat Boy
ThunderLotus - Thora from Jotun
Tinybuild - Speedrunner from Speedrunners
Totalbiscuit - John Bain aka Totalbiscuit on Youtube
Tribute - Mercenary Kings
YoungHorses - Octodad

Continously Updating
Comment if you find more

[Blue Moon NyK]

where do you enter the code?

[Selgador]

select the gear when you choose a level by jumping + UP, then Change Cameo, then add code

[Selgador]

Here is the compiled list of all i've found to date

Adultswimgames = Super House of the Dead Ninjas
Asteroidbase = Lovers in a Dangerous Spacetime (only on Xbox One and PC)
Braceyourself = Crypt of the NecroDancer
Breakfall = Starwhals
Caitbit = Caitbit
Capy = Nathan Vella + Kris Piotrowski + Super Time Force + Critter Crunch
Cellardoor = Rogue Legacy Knight
Chainsawesome = JS + Knight Squad
Denistalbot = Denis Talbot
Dennaton = Hotline Miami
Destructoid = Mr. Destructoid
Dom2d = Dom2D
Doseone = Doseone
Doublestallion = Big Action Mega Fight!
Drinkbox = Uay Chivo + Guacamelee!
Egoraptor = Egoraptor
Funhaus = Lawrence Sonntag + Bruce Greene + Joel + Adam Kovic + James Willems
Halfbot = Bullet Age
Humbledusthearts = Dust: An Elysian Tail
Ironhide = Vez'nan + Grawl
Jessecox = Jesse Cox
Juicybeast = Yowan + JP + Alex + Dom + Juicy Beast
Justinchan = Justin Chan
Koop = Gnog
Lachhh = Lachhh + Berzerk Studio
Madsoft = Andrew-David Jahchan
Markiplier = Markiplier
Mayhow = Omnomnom Game + Mayhow
Mcb = Marie-Christine Bourdua
Messhof = Nidhogg
Northway = Incredipede
Oquonie = Rekka Bellum + Devine Lu Linvega + Oquonie
Polytron = Renaud Bedard + Fez
Sauropod = Castle Story
Teammeat = Tommy Refenes + Edmund McMillen
Thunderlotus = Thora (only on PC)
Tinybuild = SpeedRunners
Totalbiscuit = TotalBiscuit
Tribute = Mercenary Kings
Younghorses = Octodad

Different from your list are:

Braceyourself
Caitbit
Chainsawesome
Doublestallion
Koop
Madsoft
Northway

[El Torso]

Vlambeer = Rami Ismail + Jan Willem Nijman (and probably also Fish from Nuclear Throne, but I already had it unlocked from just playing the game)
Note to devs : Rami's name is incorrectly spelled ! It's Ismail, not Ismael ;)