You can use a virtual machine, such as VMWare (http://www.vmware.com/). Or you can use Wine (http://www.winehq.org/).
Both systems also allow you to then test IE7 and lower, possibly using IETester. I find IETester to be excellent.
I've used them both and they worked for my needs.
Crossover sounds interesting.
To get completely deluxe, you can download Firebug Lite (http://getfirebug.com/firebuglite), which lets you inspect the DOM kind of like Firebug. It's not quite as nice, but a good augmenter for Developer Tools in IE.