Madden 94: LRLRY and LRLRA (I believe? some form of LRLR and A/B/X/Y) to play as the Panthers or Jaguars. Both teams had perfect stats.
Madden 95: BARBY got you some classic version of the Cowboys
Sonic 3: up, up, down, down, up, up, up, up during the opening sequence unlocked a debug mode of sorts letting you jump to any level, get all emeralds, spawn infinite coins, etc.
NFL Blitz: A, down was FG % and B, up was late hits (or the other way around....never really understood what the late hits cheat did but I always entered it).
Konami code for lots of games
Not a cheat code, but LARA is the code to save Lucca's mom (well, her legs) during the red portal sequence. XABY for the security code in the factory in the future.
Glitteringprizes, thereisnocowlevel, allyourbasearebelongtous, operationcwal....various Starcraft/warcraft cheats for gold, win scenario, instant upgrades, etc. Can't remember what each of these did.
DNKROZ and DNSTUFF in Duke Nukem 3d were god mode and all weapons/ammo.
player.modav (attribute) ### in Fallout 3 let you alter your stats. I usually did player.modav carryweight ### since I always wanted to be a hoarder in that game. You can also edit skills like lockpicking, science, etc.
In Super Metroid, to do the murder beam glitch--charge weapon, press start, select spazer, move cursor to hi jump boots, press left+a at the same time to turn on plasma simultaneously with spazer. It's pretty buggy though and doesn't have many practical uses. You can also charge up a shine spark in a super short distance by tapping A during the correct running frame--you can use this to do a shit ton of damage to bosses and boost through areas you aren't intended to. There's really a ton of fun glitches/exploits in the game.