the best SIMPLE p2p software, u know, to download a single song/a small program, which i recommend are:
Frostwire, Ares, eMule
If you want the real deal of p2p downloads, where you can download an artist's FULL ALBUMS or big full programs or a FULL SERIES of movies, you should learn how to use torrents, they are explosive !!
best torrent websites (where you get your torrents which you open with the torrent client which im gonna mention next) :
1) http://www.isohunt.com
2)Mininova : The ultimate BitTorrent source!
the best torrent client which with you open the torrents you downloaded from the websites mentioned above, in order to initiate the real download, you can download it from here :
Torrent - The Lightweight and Efficient BitTorrent Client