If you’ve ever asked yourself which distro is right for me? then you might be interested in trying them all.

But I have some further suggestions:

• {Free,Open,Net,PC-}BSD
• Solaris, OpenSolaris, Nexenta
• ReactOS
• Plan9
• Minix
• SkyOS
• GNU Hurd (heck why not )

I once tried quadruple booting Gentoo, WinXP, Solaris and OS X, but the latter two conflicted with each other, so I had to pick one of them at a time.