If you want a good OS for SMP, first you have to figure out what exactly do you want to do with the SMP system? If you want to do Distributed Computing projects, etc, I'd say go with Linux or FreeBSD.
If you want an easy to use general desktop, try Win2k, WinXp, or BeOS. There are several versions of BeOS with many great apps. BeOS was made from the ground up with SMP in mind--Don't knock it till you try it. Specifically Developer's Edition v2.0 or Max Edition v2.1. If you want to use a Linux desktop, I'd stick with Debian or Gentoo (even though they SUCK to install, their method of package management & application retrieving are the best available [like BSD.]) If you hate building from Source or CVS, checkout the uPM project:
http://u-os.org/upm.html a development team which is trying to make installing 3rd party software a hell of a lot easier in Linux.
There's A LOT of OS' out there, and few people outside of engineering/research institutions know about them. Write up a checklist of your needs: Ie, OS must support internet, have a web browser, support Software RAID, whatever.