Community based spam filtering
I encounter a Captcha. (1). at every step I try to add something to the web, or I try to participate. But sometimes it becomes a headache to keep typing screwed up words to do small small things as adding comments to websites and blogs.
I have also seen questions like 2+2=? to avoid spammers, I think this one is easy to beat. And the most promising form of online spam filter that I have ever seen, is the one used on Ajaxian. Its really creative to have questions based on common knowledge of web developer community to filter spam. Everytime you try adding a comment, it welcomes you with -
Spam Question: What three letter acronym is what we use to style web pages?
Spam Question: What does the X in Ajax stand for?
This helps you in two ways! First, you easily show the door to people who are least concerned/related to your website. and Secondly, its not easy to beat as the common knowledge of the community is an increasing function of time :P