Sometimes you have to use Windows. Supporting clients in multiple-OS environments makes this a necessity for some Mac users, while others want to run applications that are available in Windows only. You can get a preview of the latest flavor of Windows, Windows 7, by running Release Candidate 1 of the new OS either in a virtual environment such as VMWare Fusion, Parallels, or VirtualBox, or in Apple's own Boot Camp. Christina did a thorough rundown of the options for running Win 7 on Mac back in January.
Of course, you may not own Fusion or Parallels, and perhaps you don't want to have to boot into Windows separately. In that case, you can download a copy of Sun's VirtualBox application (it's free) and then download a copy of Windows 7 RC1 from the Microsoft website (it's free, too!). Follow these detailed instructions or watch the video, and you'll have a free way to test Windows 7 until at least June 1, 2010. Windows 7 will shut down after every two hours of use after March 1, 2010, which is Microsoft's subtle way of telling you to buy the OS or update to a later build.
If you're a VMWare Fusion user, the VMWare team has instructions and a video available here. Want to try out Win7 RC1 with Boot Camp? There are instructions here. For Parallels 4 users, I was unable to find a good set of instructions, so if any TUAW readers can provide me with a link, I'll update this post.
How is Windows 7 RC1 as an operating system? It's faster than Windows Vista on my 3 year old iMac, and some features look surprisingly Mac-like. I'll leave it to you as an exercise to see how much you like or dislike it.