All of them. You might be better off choosing an open-source project based on your interests and background rather than looking for a project that's particularly desperate.
Well, then go on - list your interests (or skills).
And please don't pick a project that's desperate for people -
It's a classic case of the mythical man month.
Most of the time the core folks will find bringing up a new member up-to speed as a distraction, particularly when there's no commitment from either side to keep working on it (compared something like google summer of code).
PS: if all else fails, pick up a new programming language & write examples/cookbook-recipes for it
All of them. You might be better off choosing an open-source project based on your interests and background rather than looking for a project that's particularly desperate.