Are you talking of qemu ???

I Know of vmware, xen, virtual server, ...

I use Virtual Box wich is stable and rock solid. I run an instance of
Linux Ubuntu 8.04 LTS on a vista machine (from work).

I have used QEMU to run Damn Small Linux on a USB key. I found it
trickier to set up the network in bridge with my connection. But hey!
It's free and it works well too.