I think Wine is one of the most promising and useful applications, especially for those who need to run Windows programs in a Linux environment. A new development release is put up every two weeks or so, and improvements are visible from each version to another.
Wine is the project which makes possible to run games like World of WarCraft, Counter-Strike, Half-Life 2, WarCraft III and so on. And Wine is also the project which makes possible for web developers to test how their web page is viewed under Internet Explorer. Not to mention hundreds of other applications which work very well or well enough with it.