Red: for being evil in the good sense.
Green: helping someone mellow out or relax, especially if they were really stressing.
Blue: loyalty to a friend, partner, family member, etc. even though it was really hard.
Purple: helping someone even though it could backfire and end up hurting the helper. A bravery sticker, if you will.
White: awesome-ness in general.
Pink: making someone feel loved.
Orange: being the "light at the end of the tunnel" or really cheering someone up.
Rainbow: being open-minded towards all sexual orientations.
Gold: an utterly amazing act, like rescuing someone from a burning building or electing a homosexual woman as Prime Minister (good on you, Iceland).
Silver: making someone feel really special or important, or both.
Sparkly: letting your talents shine without being afraid of what people think.
Random Fuzzy: being so effed up but so damn awesome because of it.
Black: can only be given to one person at a time...trust me, you'll know if someone deserves your black sticker. :D (and no, it's not like the black dot used by pirates)
Platinum: unofficially saving someone's life. Trust me, you'll know if it ever happens to you.
Polka-dot: helping someone have fun.
Well, there they are folks. Use them wisely. And often. :D
Oh, and Ellen DeGeneres and Janeece Edroff get gold stickers just because they're that damn amazing. :D