Great for all designers, but especially helpful for those designing on Macs with no access to Windows browsers. http://ipinfo.info/netrenderer/ 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. Then there's BrowserCam. Not free, but you can get a free 24 hour Evaluation: http://www.browsercam.com/
Get yourself a copy of Firefox and Opera. If you're on a Mac, you should have Safari enabled by default. For Linux users, make sure you're using the KDE distribution and use Konqueror as well. If you're on Windows, you'll have Internet Explorer by default, and can also download Safari 3's beta for Windows as well. For the rest, go with www.totalvalidator.com
If own a mac, you don't need the above links. It can be made into the complete testing platform for Windows, mac and linux by using some virtualisation software called Parallels; http://www.parallels.com/ There's also VM Fusion, which is free I believe, but it's beta quality software.
Another tool I use and works good is Adobe Browserlab although you can't test it at different resolutions
... and for Windows users there's VirtualBox (free). I use it to run OSX and Ubuntu (and various browsers) inside Windows 7.