Rewards
PlayStation trophies, Windows Live achievements, Steam achievements and Kongregate badges.
PlayStation Trophies
Windows Live Achievements
Steam Achievements
Kongregate Achievements
A brief overview of the different reward systems:
Playstation Trophies
Playstation has a trophy system where you earn trophies graded bronze, silver, gold and platinum for performing in-game actions. The harder the thing you do (whatever it may be) the better trophy you get. Platinum trophies are exceedingly hard to get as you need every other trophy in a game to get one!
Windows Live Achievements
Games for Windows and Xbox Live use the same system for achievements. When you do something cool in-game you unlock achievements worth points. The more points the harder the achievement was to get! Most games have 1,000 points up for grabs, but some games have pushed the total past that!
Steam Achievements
Steam has their own achievement system. Similar to the Live system except it doesn't bother with points. You do cool things, you unlock little pictures. Achievements are awesome!
Kongregate Achievements
I shouldn't really call them achievements. Kongregate do badges! There are four types: easy, medium, hard and impossible. It's similar to the Playstation system, except you don't need all the other badges for a game to get the impossible ones (but you probably will in the process!).