Metal Gear Solid - PSX - Let Meryl die, you get stealth, help her live you get unlimited ammo
MGS2 - PS2 - Collect X amount of dog tags and you get the above
MGS3 - PS2 - Kill bosses in certain ways - get different camos and weapons
GTA Series - Do X amount of taxi, vigilante, fire, or EMS missions - Get infinte sprint, fireproof, etc...
Boulder's Gate 2 - I believe if you just beat the game, you get 1 unlockable character, and with a code you can then unlock another
Mortal kombat: Shaolin Monks - find various hidden items, you get up to 5(i think) unlockable characters.
Resident Evil series - Beat certain time limits, get infinite rocket launcher or various other infiinite weapons and costumes.
I can go on, but I don't think I can ever list the majority of games. These were skimmed off of the top of my head and they involve similar points to how you get acheivements today. I mean, you do X amount of something or find something and nowadays you'll get text says 'Yay!', then you simply buy the next weapon or something to that effect. You don't earn anything, you either save up for it or it doesn't exist.