You just enter your URL, check the browsers you want to see, choose extra output options like screen width, color depth, and Javascript version, and then click "Submit."
Your job will be added to a queue so that all of the browser screenshots can be generated, so waste some time for a few minutes. When the job is complete, you'll have a group of thumbnails (one for each browser screenshot).
Clicking a thumbnail will enlarge the screenshot and give you information on the computer and browser that rendered your site. Clicking the screenshot again will enlarge the picture to full size.
Browsershots supports Firefox 3 (on Linux and Windows) and IE 8, but not Safari 3.
[Thanks Jonathan!]
