Check KDE - Its a DE and collection of various software. There are mentors available, so its great for beginners to get started.

http://kde.org/community/whatiskde/ http://community.kde.org/Getinvolved/development

I'm actually a KDE user: I'll check that link.

Start with the PH7 engine which is an open source embedded PHP engine (useful tool to add web interfaces to your C/C++ application). It have a very nice and clean documentation and readable code. Here is the URL: http://ph7.symisc.net/

well, I've played around with some Firefox code, but to be honest its pretty messy. I'd say if you want a project you can fairly easily understand and submit a patch for look at webkit, the code is clean and straightforward (http://www.webkit.org/building/checkout.html) though the docs are basically non-existent (if you like docs, personally I think clean code beats documentation any day)

I have read good things about Webkit, it looks a huge project: but I'll check it out.

Similar situation here, at the moment I've found interesting cppcmms (web framework), odb (orm) and ultimate++ (cross ui framework)