Windows 95 [486 & 14MB RAM (with the Winsock2 update)] and up, including Vista and x64 Professional. It also has varying success on Linux and OS X installs. See website FAQ for more details.
µTorrent is an incredibly tiny, yet full featured .Torrent program. It is free to use, although, like most shareware, the developer does offer a donations page. Unlike most shareware, there's no malware included (to my knowledge).
The Bottom Line
A small and functional torrent program that generally works pretty well; just bear in mind the usual caveats that come with the territory.
Torrent Program Of Choice
µTorrent is an incredibly small, full-featured torrent program that has quickly, and efficiently replaced my older torrent sharing softwares. Hell, with the standalone .exe being a measly 170kB, how could I complain about using a torrent software that I can fit on a floppy and run it from? (Currently, I have it running off my USB key, so I can take my downloads with me.)
It's definitely a useful program to have, and it's incredibly easy to use, but it does have some drawbacks (as minor as they may be); it would be nicer to have an easier method to add your own search engines, and it does have it's problems with certain systems. I've been lucky so far, and have not had a problem with it, but nothings perfect, right?
I should mention that there is a whole issue behind this software is one of some controversy. The author of the software happens to work for a company that had a splinter of the organization split into an anti-P2P software programmers, and the fact that it's a closed source software has lead some people to think that the author may be hiding something nefarious, so I guess it's really up to you whether you believe the suspicions of some people in the P2P community, or the author who says it's just a side project, and has no affiliation with his job. I haven't noticed anything out of the ordinary, but hey...
Obviously, this review can bring up the issue of legality, and really, I'm not naive enough to think that saying anything about it is going to make it stop...trust me, I'm as sick as anyone else with all these anti-piracy ads, but you know, just download responsibly (preferably without any rampart illegalities), and make sure to keep your virus/malware software up-to-date...
"Rampart Illegalities?"
Having only recently gotten involved in torrenting, I certainly can't say I've tried a lot of programs that are out there, but I can say that so far, µTorrent has certainly been a fairly nice program to use, especially considering it's basically small enough to fit on a 5¼" floppy disc. I'm using v1.8 now, and while I haven't tried some of the things Matt mentions in his review, I do find that certain features are quite nice, that I haven't seen elsewhere. Simple things, like having the option to remove the torrent without deleting the .torrent and the data, if I'd like. And the ability to select only the files I want (I haven't found a way to do that in Azureus or Xtorrent, though maybe I just wasn't paying attention.) I also like being able to see which pieces and piece blocks are downloading, as a way of micro-tracking just how the download's doing. There's some things I don't understand or can't find, like whether Update Trackers is the same as Find More Sources from the old P2P program Kazaa, but I'm also too lazy to read help files; happily, I'm also too lazy to do anything more than click download and walk away for three days, so it works out in the end. I have found that Firefox tends to shit itself sometimes while the program runs -- embedded videos not working, pages refusing to load, leading me to eventually reboot once or twice -- which may or may not have anything to do with the program itself, or its use of bandwidth. But that's the only thing I really have to complain about.
Put simply, this is a handy little utility that has allowed me to do exactly what I need to with it without hassling me with needless complexities and resource-sucking overhead. And it hasn't destroyed my computer yet, either, despite the aforementioned Firefox problems. It's free and it's small, so yeah, you might as well go ahead and get this one, bearing in mind the caveats Matt mentioned as well.