Ask HN: Where can I start contributing to open source efforts?

I have some spare time at my disposal and am looking to contribute to open source projects. I am not really sure where to start. I've browsed through the "Explore Github" trending section, but I haven't had much luck finding anything that I would be best suited to offer whatever I might be able to.

I largely specialize in C and JavaScript (especially node.js), but I also suffer a little from imposter syndrome and I think this would help me combat that some, and to learn and grow and improve.