Two things. Games, which is hit and miss but I'm still deeply impressed by the wine devs. Civ V runs amazingly well and it seems smother than the native OS X version, though that impression might be influenced by the wine version on Linux supporting mods, unlike the OS X version.

Secondly to compile Python projects to .exe files using Pyinstaller. Wouldn't use this for paid projects but for fun side-projects it's great.