At MIX09 Microsoft announced the availability of a new product named Expression Web SuperPreview (currently in beta). It basically allows designers and developers to compare how a web application renders in different versions of the IE browser and fix issues caused by incompatible elements. Great tool to have. See detail at the Expression Web team blog.
As Bill Staple showed in his MIX09 keynote demo, Expression Web SuperPreview supports cross-browser testing for browsers such as Firefox 3.5, Safari 3, Safari 4. See a nice blog by Long Zheng at istartedsomething.com.