I have to suggest Knytt: http://appdb.winehq.org/objectManager.php?sClass=version&iId=6849
It's a simple 2d platformer. While it's not impressive in the sense of "look at all the amazing stuff wine must be doing for this to work", it's a thing that you can just download and run (with any version of wine since 1.0). Unlike some other games, it should be immediately accessible to everyone. There's a nice tutorial you can run through in a minute or so.
It was written using multimedia fusion, meaning a native port is impossible without a complete rewrite.
Also, it looks and sounds really nice.
Just, uh, don't press escape when it's in full screen mode.
Vincent Povirk