While Microsoft isn’t the final arbiter on whether an application is “compatible” with any given version of Windows (see: http://blogs.msdn.com/b/cjacks/archive/2010/01/12/what-does-it-mean-to-be-compatible-with-windo... ) a few years ago we launched an initiative to help reduce a bit of the burden of looking up every app you care about by aggregating some of that data into a single website, which you can find here:
http://windows.com/compatibility
For Windows 8, we ended up branching it, because there are all kinds of people who are still furiously working to finish up Windows 7 migrations before April 8, 2014, so we didn’t want to take that down, and at the same time we also wanted to talk about Windows RT, so in case you haven’t noticed this (I’ve talked to a few people who hadn’t) we have a dropdown now which lets you choose the platform you are targeting:
Hopefully this will be helpful as device and operating system heterogeneity becomes more commonplace!