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.