I have just started my first project on CodePlex. The project is aspNETserve, a ASP.NET web server written entirely in C#.
The purpose of this project was mainly for me to gain a deeper understanding of ASP.NET. In the short time that I have been working on this project, my knowledge of ASP.NET has grown by leaps and bounds. I plan on writing about some of the specific things I learn while working on this project.
On the topic of CodePlex… The command line source control tool is a little awkward to use, but fortunately they will support Subversion soon, so I can just use SmartSVN as my source control client.