Hmmm. Did my bit for karma this morning. It's a topic that gets talked about a lot on the Singletrack forum, particularly in terms of car drivers, crashes, insults and broken bones. Since I don't believe in reincarnation, I can't take the traditional view of karma (what you do in this life affects your reincarnation state), but I guess now people use karma as a quick version of the 'do unto others as you would be done by them' idea. I do believe that if you can go out of your way a little to make someone happier, in whatever way, that you can be happier by that, and that they might spread that little bit of happiness on to others, which can't be a bad thing in the long run.
Today it was as simple as passing on a book. It hadn't cost me anything, didn't have an obvious owner (lost property), and I'd read it and enjoyed it. So I passed it on. And what made me happier in return was the guy said he'd do the same when he'd read it.
So maybe karma does exist. If you use the quantum weather butterfly analogy, maybe one small act makes the world a better place overall. I don't know about that, but it made me a touch happier this morning.