The browsers themselves. For IE though you should consider either a virtual image, the Tredosoft Multiple_IEs or IETester.
good blog post here: http://justfreetemplates.com/blog/2009/09/18/7-ways-to-check-cross-browser-compatibility.html but will depend on what browsers you want to check and what OS you currently have. I suggest IEtester to test IE with - versions 5.5-8, but it only works on windows.
Thanks, spyka, that's a very good resource. Hopefully in the future we won't need any browser checkers, as maybe all the browsers will follow W3C specifications.
You will never get away from the need to do multi browser testing. Even if everyone was W3C compliant (which I doubt they ever truly will) you still need to check what a site looks like when common browser add ons like FireFox's AdBlocker is installed and running - no one wants their advertising revenue blocked but you do still want the site to look good if it is. W3C isnt going to set a standard for these types of plug ins so you will need to have the different browsers with the different plug ins to check.
Multiple IEs is good... but just not exactly correct for IE6. I've tested using Multiple IEs version of IE6 and IE6 installed on an older PC and there are some slight differences. The best way is to use the actual browsers i find.