Recs.
Updated
Deluge is free, open source and cross-platform BitTorrent client. Deluge aims to be a lightweight, secure, and feature-rich client.
SpecsUpdate
Pros
Pro Full-featured
Deluge covers basics such as Prioritization, Selective Downloading, DHT, PEX, LPD and Magnet URIs. It includes MSE/PE for security. It has graphical, web and command line interfaces. Has UPnP, NAT-PMP, NAT traversal for automatic router configuration. Has UDP tracker data transfer protocol support. Even covers IPv6 and proxy servers.
Pro Portable
Deluge can be run without having to be installed. It can be put on an external storage device (such as a USB drive) and run directly from it. A portable version of Deluge can be downloaded from SourceForge.
Cons
Con No super-seeding support out of the box
Super-seeding is an algorithm used to allow downloaders become uploaders quicker. Deluge lacks support for this functionality out of the box. However, the ltconfig plugin can enable this setting: http://forum.deluge-torrent.org/viewtopic.php?f=9&t=42887.
Con Can't "force start" downloads
There's no way to manually override settings for download queuing that's available in other torrent clients. For example, if there are 2 files in the download list and the software is configured to queue just one at a time there's no way to force a download of both without changing the settings. A workaround plugin can be found here.
Con No built-in broadcatching support
Broadcatching is a method of automatically downloading content as soon as it's available. Broadcatching in BitTorrent clients is usually implemented via RSS. Deluge requires manually selecting each download by default. Fortunately, there are multiple plugins available that add this functionality with the possible caveat of plugins not having the same quality standard as the client itself.
Con No web seeding suport
Web seeding support allows downloading file parts from an HTTP source alongside the swarm. It's usually used for long-term seeding and easing some of the load on web hosts that supply direct downloads by offering a BitTorrent download as an alternative while still ensuring that the file is always available in its entirety regardless of the swarm. Deluge lacks this functionality.