Search on Google Code Project Hosting or SourceForge, or even better still, do a search for C++ projects on Ohloh. Unfortunately you cannot do searches on code quality, but at least you can see a report on the projects page to find some with good quality metrics and relatively small codebases.
Some projects that I've come across with good code quality and in c++:
- Relatively small codebase:
- Code::Blocks
- InkScape
- Mona OS
- VLC
- Not that easy to get into:
- Blender
- Chromium
- SpiderMonkey
- Never really looked at but could be interesting:
More importantly, just look around and see if a software you like and use often is developed in C++. You'll have a greater incentive to contribute and to see your changes in real-life and used by others.
You may also want to refer yourself to these other SO questions:
- C++ projects for beginners,
- Open-source project for C++ developer.
