Best overall is Ares, download it at www.download.com it offers more options than LimeWire but is a little bit slower if you can't port forward, and doesn't offer many sources, using Ares and LimeWire in a 1-2 punch system is a very good way to get the files you want. I have also heard very highly of SoulSeek.

Stay away from BearShare, and Kazaa, and IMesh they are worthless.