Since most *nixes are associated with proprietary hardware (pSeries->AIX, HP9000->HP/UX SUN Sparc->Solaris. The only way to dual boot is to use an x86 based *nix like Linux (from the 100s of flavors), BSD (of the many flavors) and even Solaris on Intel
If you don't want to dual boot and you want to experiment without messing with your machine, you can install the free VMware server and install the OS as VMs
On my Lenovo 3000 laptop I have virtualized, RHEL5, SuSE Ent 10 and Solaris 10 (intel)
There a plenty of resources to help you get started which are a google search away
HTH