First let me open by saying I do not condon copyright infringement or theft.  Companies and individuals are entitled to fair and just compensation for their work; however, US Copyright laws entitle you to backups for personal use, and there are a number of indisputably legal uses for P2P file sharing.

I recommend you check out Shareaza (DO NOT USE shareaza.com — that’s a SCAM site — use the Source Forge link below).  Shareaza is a multi-protocol P2P file sharing client; it does Gnutella, Gnuetella2 (G2), EDonkey2000, and BitTorrent.  It also includes the ability to search for files.

If you’re just interest in exchanging files via BitTorrent then uTorrent is by far the best client.  BitTorrent has a search tool built in, and you can certainly use other sources for torrents as well.

Reguardless of the client you choose, you should consider running PeerGuardian 2 fro Phoenix Labs (you can read all about it on their site).

And I recommend you consider running your P2P client in a virtual machine.