What is BitComet (A powerful C++ BitTorrent Client)?
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
Support preview of video file which is being downloaded.
Support DHT Network, download trackerless torrent!
Completely new core written in C++, stable and fast, very low CPU usage. (detail)
Ability to limit the upload speed as well as download speed. (detail)
Clean and free, without any adware or spyware. ( Please note: Spybot may misreport "Comet Cursor" )
Multiple simultaneous downloads, ability to select download files in one torrent and set file priority.)
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)
Support automatically power off with conditions.