Jon Skeet announced his intention to port JodaTime to .NET I said I would be interested in helping in the process.
NodaTime was born and I have carefully following the discussions in the groups and seeing contributed code as it's checked in.
Unfortunately a number of other commitments and the realisation that I actually have only about 8 hours a week free time (for all my interests - not just writing my own code) have so far prevented me from making any kind of contribution to the project.
Despite my lack of involvement I've learnt some interesting and hopefully valuable things from looking at the code already contributed to the project and from reading the discussions around it's development.
Based on this experience I'd recommend that any developer follows the discussion and looks at the code from new open source projects. It's a great way to see how other people work and solve problems. Of course if you have the time and opportunity to contribute also then all the better.