The site at browsershots.org is a great idea. You enter your URL, select which browsers/operating systems you care about, and they go out and take screenshots using the browser/OS combinations. This way you can see how your site looks to other users without having to maintain a slew of machines, or virtual machines.

The downside is that the implementation of this site kinda sucks. You have to wait to see your results, and the queue can be several hours and since they only keep you in the queue, you sometimes don't even get any results before you are booted from the queue. But this is currently in "alpha" mode, so hopefully it'll be getting better soon.