Why'd you want to? Doing something illegal?
I think there are programs out there with which it is a bit more difficult to trace users. Dunno their names or anything, but they work something like this: Computer A sends out a request for a file to computer B. B then sends the request on to computer C. But computer C only knows it got a request from B. It can't see where the request originated. If C has the file, it sends it to B, which in turn sends it on to A. At least I think that's what happens, but someone could probably correct me.