Introducing µTorrent. It is a pretty young client, and already it is gaining a groundswell of rabid users. I have been using it for a while and, barring an unfortunate tracker stat incident with an earlier version, am very pleased with it.
- The size of the executable is under 100 KB
- CPU and memory utilisation are minimal, as in just about insignificant
- Very fast iterations for bug fixes/new features (though this means users are all essentially beta testers)
- Enough features for most users
The only negative I can think of is that it is not open source, but probably a moot point given the amazing work done so far.