At the moment I'm using FreeBSD to learn about FreeBSD
My main OS is GNU/Linux Debian, but I wanted to try something else. So far I've had a pretty good experience with FreeBSD. Not too different from what I'm used to in many ways, so I'm doing most thing I've been doing in Debian without much hassle in FreeBSD. This includes e.g. webbrowsing with Firefox and lynx, torrents with Transmission, text processing with LaTeX, text editing with vim, news with slrn, mail with mutt, mplayer for video playback, mencoder for video transcoding and DVD ripping, mpd/sonata for audio playback, audacity for audio editing, gimp/imagemagick for image manipulation, bash for file management and all... The list is endless.
I'm also planning to deploy FreeBSD as a router, using a Soekris machine. This then will also include mail/web/print serving.