Bonus Points are a reward given by the GM to Players for in-game and out-of-game achievements.
Bonus Points are rewarded at the discretion of the GM, and should be given out very rarely.
Normally a GM would give these points out to players that go above and beyond the call. Such as excellent roleplaying, figuring out a tough riddle or doing something that so un usual, you have to just give it.
So you are saying, "Ok. So how many can I get? and what can I do with them?"
Easy, you can get upto a max of 3 points at anyone time, the points expire after the next gaming session.
After each session is over, everyone secretly votes for the best player of the night (can't vote for your self).
The GM will reward the other 2 points as he sees fit.
There is only a total of 3 points given out in one session.
What can a player use them for?
A natural 20 hit (can do this once per game)
Reroll any dice/die roll once, but have to keep the new results. (can do this 3 times)
Get out of one's own death (can do this once)
Heal half hitpoint total (can do this once)
This is what I've toyed around with in the past, and I've gotten some good results from players, as they seem to enjoy the little rewards, and have fun spending them at just the right time.