For money: a system that manages data for preparing 510(k) submissions to the FDA for new medical devices and automates the process as much as possible. From watching how much effort my multi $Bn employer goes through to do this, there must be many, many smaller companies that suffer orders of magnitude more than we do. And they'd pay a handsome amount for a tool.

-or-

I'd find out just what it is that so many people hate about their CAM software and fix it. Bonus points if I get to build hardware as well: iPhone interface to a Haas OfficeMill anyone?

For fun: a walking robot with high payload (> 1/2 ton) capacity.