Today I saw that there is a very simple but cool method to detect users of 
http://tor.eff.org/.
Of course Tor doesnt claim to be undetectable but encrypted an anonymous access to the web.
I hope noone uses this as a chance to deny access to web sites for anonymous users.
The test is pretty easy and is done via CSS and JS.
Test yourself: 
http://ha.ckers.org/weird/privoxy-test.html
[via 
ha.ckers.org]