This summer Google is sponsoring a number of open source projects with their Summer of Code 2006.
Last year Mono had a great success: a lot of fresh code was contributed and many of the students that participated in the effort continue to be contributors to the effort (see my previous comments about it).
See our page with ideas for summer projects and feel free to email me if you have any questions (or reach me on irc.gnome.org, channel #mono).
In addition to Mono, the Beagle Project is also being sponsored for the Summer of Code. They also have a great list of tasks to implement in the joyful world of C#.