Tools to test across all browsers

http://www.browserstack.com/
BrowserStack is a web based cross browser testing tool.  Has a nice feature to let you test on development server that might not be exposed to the public.

http://browsershots.org/
BrowsersShots let you enter your URL and do a browser compatibility test across the browsers you checked. Some people prefer Adobe Browser Lab because they claimed it's faster than Browsershots, and one can save his/her own set of browsers.

http://netrenderer.com/
IE NetRenderer allows you to check how a website is rendered by Internet Explorer 10, 9, 8, 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!

https://browserlab.adobe.com/en-us/index.html
Adobe BrowserLab is an online service that helps ensure your web content displays as intended. Accurately preview web pages across multiple browsers and operating systems, navigate links, and use diagnostic tools to optimize websites efficiently.

https://addons.mozilla.org/en-US/firefox/addon/adobe-browserlab-for-firebug/
Adobe BrowsersLab for Firebug

http://spoon.net/browsers/
Browser Sandbox lets you "run popular web browsers with no installs" (IE6, 7, 8, 9+, Firefox, Chrome, Safari, Opera).

http://finalbuilds.com/iecollection.htm
Multiple IE versions you can installed.

http://www.microsoft.com/en-us/download/details.aspx?id=4580
For testing in different IE versions on Windows, you can also use Microsoft Virtual PC.

Note: The above is not an exhausted list of tools you can use for browser testing.  If you have more tools you have seen and want to share, please feel free to add it in the comments, thanks!