Thanks to Gus, I took a quick tour through the MyDreamApp finalists today, and voted for two of them, Portal and Blossom. Portal is a long shot - it's basically proposing to solve multi-computer file syncing seamlessly, but it would be great if done right. See Wil Shipley's comments for why it's hard. Merging is not easy.
Blossom is a great idea to link productivity nannying with emotional appeal - work hard and your desktop plant grows! It needs a new name, but with the right design and art, I think it'd be a hit. If they took a cue from the Nike+iPod multiplayer game, we could compete on productivity - make work a game and compete to see who can build the biggest, healthiest desktop plant. Brilliant!
Today's Windows User Fun Fact: I had to use Internet Explorer to see the entries on the MyDreamApp page - Firefox didn't show the entry details when I clicked on the pictures.