Probably what most do with theirs. Email, web, iphoto, itunes. Some home photo and movie editing. My macbook gets used for work when I'm on the road. Unfortunately, I do have to boot into Windows for this one stupid program they make me use. At least I can still use my Mac.
I also use my iMac when maintaining my servers. I like that I can pull up a terminal, ssh into a freebsd or linux server and run an X-windows program without having to install any additional software, like cygwin.