I really hate to say this, but, anonymity on the net really only exists as fiction these days.  Tor has had problems with it's exit nodes for a very long time and there was a lot of talk in the "penetration testing" community about the FBI using Tor to set up stings last summer.  One can use a VPN (virtual private network) that claims to keep it's users secrets secret, but there is that incident where a member of "anonymous" had his activities reported to the FBI by the VPN provider he was using. (I believe it was the "Hide My Ass" VPN service).  Proxy servers, both public and private, but mostly the public ones, leak tons of information to other people using those networks.  Sometimes, a simple program like Wireshark is all that is needed to gather the info required to identify and track users.