MS has released modern.IE as a tool that lets web developers scan a web page and guide them on how to fix common compatibility problems. Sure it is all about MS wanting the best experience on IE across the whole range of devices (and Windows 8 tiles as well) but anything that helps is good.
MS has also partnered with BrowserStack to offer 3 months free testing on a wide range of browsers running on virtual machines. These include Win XP (Firefox versions 3-18, Chrome 14-25 etc), Win 7, Win 8, Mac OSX (Lion, Mountain Lion, Snow Leopard), iOS and Android (with a wide range of models) See the list of browsers supported.
While you are at it check out one test.