System Commander will allow you to boot from a variety of systems.. I don't personally use it, so I cant give you the particualrs...
I have multiple systems and thats how I test OS's (running about 12 PC's at home) but I know this isn't possible for everyone.  WinNT/2000/XP also will give you multi boot options.

I also know there is a device available (I saw it at TigerDirect) called "Trios" or something like that that allows for a tri-boot type system.

Just a suggestion