How to pick an open source project to join?

I am interested to join an Open Source project using .net; I would like to be able to choose between a mature project and a starting one, as well as other criteria. What do you suggest? (specific projects are welcomed).

Later edit:

I am disappointed by the arrogant comments / answers received so far. Picking an OS project to contribute to is not a simple task. I have spent many hours today doing this. My experience shows that:

few projects advertise how to contribute to them
few projects even let you contribute
a lot of projects expect you to contribute by writing docs and answering user questions
there are many projects that are "dead"
not everybody uses OS software written in .net - it is a starting community - so the criteria "contribute to the project you use" is not useful

I believe this question is far from being answered. I have carefully read the related questions, there are some good points there, yet nothing truly revealing.

Solution

I find my own interests reflected in your question, so here are some o-s projects that I am considering:

SharpDevelop - they've gone a long way, and still a long way to go :-). I think some improvements for code navigation and code refactoring a la ReSharper (or even Eclipse) would be very cool.
NHibernate - support for generics (although I think Ayende already has this covered); support tools - visual editors for xml configs, maybe a visual tool (VS addin?) that takes one of {model, mappings, database_schema } and generates the other two.