There are plenty of them. It all depends on what motivates you.
Spring has lots of them. www.springframework.org
JBoss has lots of them. www.jboss.org
Note, that at first you won't be able to commit to those types of projects, but can submit patches to them, and as you do more and more, they might let you become a committer, and in the two cases, if you become a good committer, you might even get a job offer.
Mark