Well I use HyperVM which runs on Xen to boot up virtual environments for testing and deployment. At the moment I only have CentOS images setup, but when I find some time I'll add a few more linux variants.

Though, I have to say, I'm trying to move to something else. HyperVM is so botched at the moment, it's just useless to add features to. The guy that ran the project apparently is no longer with us, so the source has been released to the public, and currently nobody is maintaing the project.