IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
If your looking to contribute, and you can program in C, then you'll be able to help many projects. If you're interested in a real ground breaker, check out Genode http://genode.org/. It's a new microkernel OS Framework, and it's the most promising I've seen yet. I personallty like working with Genode, running the FIasco.OC microkernel, but the kernels are completly modular. I belive this project might one day shake up the current OS landscape.
Why not develop a simple Android app? It should satisfy the open source requirement, give you some practical experience, and be relatively straightforward. There are lots of resources available to help you through it both on-line and in print. Plus, there is even the possibility that you could list it on Google Play and make a few bucks off of it, if you do something innovative.