If you want to learn about the concept and armsrace, here is a paper with plenty of resources (this is in a game context, though not website, although there is the most advanced detection here): http://iseclab.org/papers/botdetection-article.pdf
Here is an open source system demo'd at BlackHat Europe 2011 (that checks it is a proper browser (with DOM/Javascript/etc), also good against DDoS.
https://github.com/yuri-gushin/Roboo