No. 1: uTorrent
Or known as "micro torrent". Required low hardware profile with minimal impact to the rest of your computer's speed.
No. 2: StrongDC++
Free client for sharing in Direct Connect network based on CZDC++ and has partial file sharing, download/upload speed limiter, safe segmented downloading. Is an Open Source so you can download a source code and modify it.
No. 3: eMule
Based on the eDonkey2000 network but more features than the standard client. Is an Open Source software.
No. 4: LimeWire
Free file sharing Gnutella client for Windows, Mac, OSX and Linux. Fast and easy-to-use.
No. 5: SoulSeek
Good P2P software for non-mainstream music. It's virtual rooms allow you to meet people with the same interests, share information and chat.
No. 6: Azureus
A Java language client. Multiple torrents downloads/upload with download speed limiting and advanced seeding rules.
No. 7: Shareaza
Multi-network P2P supporting Gnutella2, Gnutella, eDonkey2000 (eMule), HTTP, FTP and BitTorrent protocols. Most sophisticated file sharing system on the network.
No. 8: Ares
Powerful library organizer, built-in audio/video player, files sharing chat rooms. Automatically finds more sources and downloads files from many users at once.
No. 9: BitComet
Powerful, fast and very easy-to-use. It supports simultaneous downloads, download queuing, selected downloads inside a torrent package, fast-resume, speed limits, port mapping, peer exchange and IP filtering. BitComet available in 43 different languages and the current preview release comes bundled with the BitComet FLV Player. eMule plug-in available and Media Player Classic (MpcStar) download link. Personal blog provided.
No. 10: KCeasy
Windows P2P free and open source file sharing software which uses giFT. Its plugins allow giFT to support different kinds of file sharing networks. Available for Gnutella, Ares and OpenFT.
No. x: Thunder (XunLei)
This is another supreme downloader call Thunder or known as "XunLei". eMule download available and very high speed. Chinese language interface and network domination. High impact to the rest of computer including own PC.