I've been mucking about with a little play app that solves sudoku puzzles. I know the first release is a bit naff but I published it on codeplex today. I needed some kind of source control before I evolve it further.
As a problem domain, it's quite nice for object-oriented programming and I've wound up using quite a few C# language features. I think the next step is to implement a Data Model - View - View Model pattern before trying out WPF animation.