I've just posted a tutorial for a fully fledged particle system over at DieHard Wolfers. By default it acts as a replacement for the normal rain and snow, but it can be used for just about anything you want.
http://diehardwolfers.areyep.com/viewtopic.php?t=6756
http://diehardwolfers.areyep.com/viewtopic.php?t=6756