Realistically would any programmer want a novice to contribute other than documentation? Edit: I have already been doing the things you are saying (learning from everyone) but I feel I am not progressing fast enough.
On just about any project basic bug fixes would be good. You know the easy noncritical stuff that tends to get overlooked. Fix up the slapdash UIs open source apps tend to have. Tests, beat on a library's interface in a way that someone too close to the code base might not think of. Depending on the app, data import\export might be a good place to start. It rather depends on one's area of interest.
I want to focus on deliberate practice by choosing projects that is relevent to my weakness. Right now I want to master C. Do you know a good projects for this?
Open source projects written in C. That is like shooting fish in a barrel made out of fish, on a floor made out of fish, with a gun made out of fish, with big fish for bullets.
You might try Redis, MongoDb, the Linux kernel, One of the BSD's kernels, Python, mongrel 2....
Yes. Working on the same problem space for a long time can get boring, and a fresh face willing to take it on makes it worth the time investing in code reviews and mentoring to provide continuity for users. Professionally at least, I would happily pair program with an intern for 6 months nonstop to bump the bus number on all my responsibilities.
Nothing wrong with documentation. I would do it just not the only thing I want to be doing. Do you know a good road map to make substantial contributions?