Great for all designers, but especially helpful for those designing on Macs with no access to Windows browsers.

http://ipinfo.info/netrenderer/

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IE NetRenderer allows you to check how a website is rendered by Internet Explorer 7, 6 or 5.5, as seen from a high speed datacenter located in Germany. Just type in a URL in the field above and try it out - it's free!
Another one is Browsershots: http://browsershots.org/
This one isn't immediate, your 'job' goes into a queue, but it really doesn't take that long. It does give you a big choice of the OS's and browser configurations such as screen resolution, flash, javascript, color depth, etc.

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Browsershots makes screenshots of your web design in different browsers.
It is a free open-source online service created by Johann C. Rocholl.
When you submit your web address, it will be added to the job queue.
A number of distributed computers will open your website in their browser.
Then they will make screenshots and upload them to the central server here.
Then there's BrowserCam. Not free, but you can get a free 24 hour Evaluation: http://www.browsercam.com/

****** If you have new resources, PM me and I'll add them, this thread is now locked ****