VirtualBox is an open-source virtualization tool for the Mac. It is basically a tool that allows you to run other operating systems within Mac, without having to restart your system or booting into another partition. It is the only major open-source alternative to VMWare and Parallels Desktop, so it is always worth a try.
VirtualBox currently runs in Windows, Mac OS X, and Linux, among others. It allows you to virtualize Windows, Linux, DOS, Solaris, OpenSolaris, and other operating systems.
There is so much to say about what VirtualBox can do for you, so I will let you experience most of it first-hand. I will tell you about my favorite features, though. Perhaps, the most important "feature" is that this is a free tool. The Mac version is very easy to install and setup and I haven't really found any problems with it. Recently, two important features have been added that I think are worth mentioning. The first one is support for 64-bit operating systems. The second one is the ability to render DirectX8/9 content within your virtual machines. This paves the way for gaming on VirtualBox.
The usual set of features for a virtualization tool can all be found in VirtualBox. There are good networking options, support for snapshots, and Mac OS X/Virtual machine integration.
When you finally decide on which virtualization tool you want to use, the decision will probably boil down to personal preference. Thus, I recommend you try this, VMWare, and Parallels before making up your mind. Finding the perfect tool for you can often save you loads of time.