Heh, I think that I've used almost a dozen different ones so far... Linux, FreeBSD, OpenBSD, NetBSD, MacOS X, DragonFlyBSD, AmigaOS, BeOS, etc.
I really liked trying every OS I could get my hands on some time ago, but nowadays I prefer to have a stable one on the machine I use primarily and that's why I don't dual boot anymore, but if I have to use another OS, I put it on another computer.
I just don't like virtualized environments or emulators to run an OS, it defeats the purpose.
Currently I run Windows Server 2003 x64 and my brother Windows Fundamentals for Legacy PCs.
On an almost unused laptop I put SkyOS, just to try it out.