Wine, a project which runs Windows binaries on Linux, OSX, Solaris, and BSD, as an alternative to running a copy of Windows in a virtual machine. Hence, W ine I s N ot an E mulator. Instead, it uses native and reverse-engineered libraries to mimic the functionality of Windows
The project keeps a database of which programs run well under Wine, called the AppDB, rating an applications stability from Platinum (extremely stable) to Garbage (unusable). Wine also hosts a wiki detailing generally with using Wine and running specific applications (such as Photoshop).
(Note, however, that Wine is not designed as a substitute for .NET - instead, use Mono).