I recommend FreeBSD. :-)

In all seriousness, every open source project can always benefit from more help... provided that it comes from someone who is interested in the project and is qualified to work on it. Moreover, people who come to a project saying "I want to get involved in open source, and I've picked this project" are usually nowhere near as successful as people who come to a project saying "I've been using this for a couple of years, and I've noticed these bugs... want some patches?"

So my recommendation is to simply start playing with open source software, and when you encounter something you think is wrong or could be improved, fix it.