My favorite is edonkey2000 check it out at guess where edonkey2000.com
It kicks ass. Automatically DLs from the fastest person with the file you want, and you can UL it as you DL it. Practical good of this: Say you want a 600 meg file, but the dude who has it is on a modem. However, someone with a cable has been downloading it from him for a while, and already has 400 megs of it. You will automatically get the first 400 megs from the guy with cable, and only have to get the last 200 from the original guy on the modem. Score! small user base so far, but weekly updates.