Chris Craft's 30 WM Apps in 30 Days
I know that most of you have already seen this, but if you haven't, it is DEFINITELY worth taking a look at.
Chris decided to build 30 Windows Mobile apps in 30 days...and a LOT of these apps are really useful! Go ahead, take a look!
My favorite, I think, is the Mobile Capture app (day 5). Give it a shot :)