Most php bulletin boards have it built it, I suspect we do also but it's not activated.
Basically, each user has a "reps" (reputation points) bar under their avatar. If you like a post of someone's, you can click an icon and rep them, and leave a comment that they can read in their profile. This adds to their points and the bar fills up. If you dislike their post, you can neg them (negative reps), likewise leave a disparaging comment, and detract from their reps. It's a lot of fun, especially when someone is just plain dumb and gets negged so much that their bar is in negative.