Up until two months ago, had I been asked the question "Which is the open source project that you are least likely to ever contribute to?", I would have, without hesitation answered OpenOffice.org.
But since the LibreOffice fork the answer to that question is different. The OpenOffice people that initiated it made a few nice changes in order to break down the barriers.
* dropped the annoying contributor agreements
* restructured and migrated the code-base to git repositories (about 20, because of performance considerations apparently) on freedesktop.org
* started maintaining a very active wiki with potential easy tasks for newcomers and are very welcoming and helpful on the mailing list and IRC
rest here