I'm currently using µTorrent at my company for transfering large files over our somewhat flakey WAN. We have 50 remote sites all using µTorrent daily and we're currently using VNC to use µTorrent on their end. Our remote sites use a variety of NT4, Win2000 (Pro and Server), WinXP and Win2003 so we'd be testing on all of those OS's. Also, multiple browsers used IE 4, 5, 5.5 and 6, Firefox and Opera.