VirtualBox on Snow Leopard
I've heard some news about troubles with VirtualBox when running on Snow Leopard. It was too late for me as I've just upgraded a couple of my systems. So I installed VirtualBox and I created a sample Ubuntu 9.04 system from scratch. Everything ran smoothly. Network and USB access worked as expected too. I have no idea if there is trouble running a VM you created before upgrading to Snow as I have none.
I used to be a fan of VMware's free VMplayer. Unfortunately there seems there is not a free version OS X. VirtualBox appears to do a nice job too. There is a quirk though, USB devices need to be registered to be used by the VM before you turn it on.