This is a portable bittorrent client with basic features along with support for extensions. It is currently being actively developed on Windows 7, Debian and OSX 10.6.8. The most recent releases (9-dec-2012) are 0.2.2 for Windows 7, Debian 6.0.6 and Ubuntu 12.04, all 64-bit. I am not planning 32-bit releases for at least the next week. OSX will be released in a few days.
Extensions are supported, although not very documented. A simple server is available for download that is compatible with how the uTorrent WebUI adds torrents. Therefore, Firefox extensions such as BitTorrent WebUI+ work fine for adding torrents (http://127.0.0.1, port 2999, username/password blank.) Extensions are installed from the Tools menu and are actually 7z files with the .btx extension.
Baretorrent - 64bit software