BitComet:

BitComet is a p2p file-sharing freeware fully compatible with Bittorrent, which is one of the most popular p2p protocol designed for high-speed distribution of 100MB or GB sized files. BitComet is a powerful, clean, fast, and easy-to-use bittorrent client. It supports simultaneous downloads, download queue, selected downloads in torrent package, fast-resume, chatting, disk cache, speed limits, port mapping, proxy, ip-filter, etc.

Features
*Clean and free, without any adware or spyware. ( Please note: Spybot may misreport "Comet Cursor" )
*Completely new core written in C++, stable and fast, very low CPU usage. (detail)
*Multiple simultaneous downloads, ability to select download files in one torrent and set file priority. (detail)
*Ability to limit the upload speed as well as download speed. (detail)
*Support DHT Network, download trackerless torrent! (detail)
*Intelligent Connection Optimize, Auto Optimization for different connections, runs well using all default settings.
*Intelligent Rate Control, optimize the upload distribution in order to get the max download rate.
*Intelligent Disk Cache, decrease the potential damage to the hard disk when high-speed downloading (>500KB/s) by decrease the read / write frequency, extra *memory usage is the cost. (detail)
*Intelligent Disk Allocating, no long-time disk allocation, also decrease the disk fragment to almost zero.
*Intelligent Hash Scan, no time-consuming scanning when seeding / resume. (detail)
*Firewall and NAT Traversal, users behind differect NAT can now connect to each other (detail)
*Auto config ICF (Internet Connection Firewall) and ICS (Internet Connection Sharing) in Windows XP
*Auto config port mapping in router (UPnP support is required in router, Windows XP is required). (detail)
*Ability to block IP temporarily or permanently, compatiable with eMule ipfilter.dat.
*Ability to chat with other downloaders ( disabled by default )
*Compatiable with Windows XP SP2 TCP/IP restriction (detail)
*ZIP package is also provided, which runs withoust installation.
*Using only one TCP listening port. (detail)
*Support Multi-tracker, and utf-8 extension, UDP tracker Protocol v2.
*[Please Note]: It is reported that Zone Alarm 5.0 conflict with BitComet (and other p2p maybe). (discussion here)