Today’s web suffers of a terrible plague of spam. Each form is a way for spammers to send their content : blog comments, forums, wiki, … I call this « Form Spam ».
To fight this, most of webmasters out there use a « captcha » : a picture with text inside that people must copy. It’s annoying, it’s boring, it’s hard to read (see some examples) and it’s the worst thing ever for people with bad sight. Worst of all : some robots can now read them !
For almost a year now, I’ve been using a hand-made solution called « Invisible Captha Mechanism » that works perfectly : I have approximately one spam/month in my comments. (I had more than 200/day before ! I also solved completely the trackbacks spam problem) No captcha, no known false positive, no annoyance and easy to implement. For me, it was perfect but I was just to lazy to document it for you before today (and you know how much I like to fight spam).