Browsershots : Test your website in different browsers
browsershots.org
browsershots.org
http://crossbrowsertesting.com/
The service allows you to pick your platform and installed browsers and then do whatever you wish on a virtualized instance. You can get 5 minutes free, otherwise there are paid options.
That said, its nice when you don't have the option to access your page through some different OS and browser combos, plus they will let you download all the screenshots in an archive.
The way it work is, you post your link and then remote browsers get the url and shortly thereafter, they take a full length screen shot of your web page.
Paying gets priority placement, but even with sites that require javascript, if the page has JS in the onload, then that will run before the screen grab if there's enough time.
I've been doing a lot more web testing for a project this week, this might save me some serious time hunting down problems.