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http://virtualbox.org

I wouldn't have recommend it about a year ago but nowadays it's caught up with others quite nicely feature wise, is very fast, supports snapshots and USB, and is completely free.

I second this! I wrote a post ( http://goosey.posterous.com/the-good-news-is-you-dont-have-t... ) recently about this development strategy. It has many advantages beyond the "you don't have to worry about hosing your laptop" one, for sure!

I've been using (Oracle) VirtualBox and am happy with it. I'm on Snow Leopard and have 4GB RAM.

/Second VBox. I'm using it on Ubuntu to play around with Arch, Nexenta, OpenSolaris, and WinXP. It just works.

I'm pretty happy with VMware Fusion.

http://www.vmware.com/products/fusion/

I like VMware fusion provided you use the isolated mode. Trying to "fuse" windows 7 into OS X is the only thing thats crashed my macbook so far.

Just an FYI, VirtualBox images are not easily transferred to VMware, and vice versa.

If you're only ever on a Mac, I vote VMware Fusion, but if you're going to change vm host environments, I'd suggest VirtualBox.