The easiest and most obvious answer is to scratch your own itches, as it were. Find a bug in a piece of software you're using? Report it, or better yet, fix it. Poor documentation? Improve upon it. Missing feature? Try to implement it.
If the software you use has no immediate flaws, you could ask your friends about their itches and try to scratch them.
If that's not an option either, then there's always the internet. Look around on sites like freshmeat or hacker news for new and potentially interesting projects. Offer a hand where possible.
