Unity is a Mac OS application we used to make The Graveyard. Projects made in Unity can be compiled for Windows. And through an additional package to Wii (which we hope to use in the future).
Its nicest features are asset management and the fact that it knows you want to make a game with it, so a lot of systems that are common in games come prefab with the editor. Programming is done through scripting (Javascript, C# or Boo). These scripts are called Behaviours and you attach them to objects. Exposing a script's parameters in the editor allows for realtime tweaking. But the application has distinct “play” and “edit” modes.
nik