I am looking to roll my own simple web stats script.

The only major obstacle on the road, as far as I can see, is telling human visitors apart from bots. I would like to have a solution for that which I don't need to maintain on a regular basis (i.e. I don't want to update text files with bot-related User-agents).

Is there any open service that does that, like Akismet does for spam? Or is there a PHP project that is dedicated to recognizing spiders and bots and provides frequent updates?

To clarify: I'm not looking to block bots. I do not need 100% watertight results. I just want to exclude as many as I can from my stats. In know that parsing the user-Agent is an option but maintaining the patterns to parse for is a lot of work. My question is whether there is any project or service that does that already.

Bounty: I thought I'd push this as a reference question on the topic. The best / most original / most technically viable contribution will receive the bounty amount.