Yeah, I've used vmware, although not on a daily basis. I tend to use it to try out LiveCDs. It was easy to install on Ubuntu Edgy (it's in the multiverse repository with the package name "vmplayer").
It's pretty good, although any of these virtual machines are pretty heavy on memory, so if you've got less than 512 MiB of RAM you might struggle a bit.
qemu is free/libre but very slow unless you get hold of the proprietary "accelerator" kernel module. I had a lot of trouble installing this though, and gave up (that was a while ago now, so it might be easier these days). You can use the qemu-img tool to make image files for use with vmplayer
I've not used parallels myself, but the folks on the Linux Link Tech show seem to like it a lot, as do the Security Now netcast folks.