Question: Programming is learned by writing programs. But code reading is said to be another good way of learning. I would like to improve my unit-testing skills by reading, examining real-world code. Could you recommend any open source projects where the source is extensively tested with unit tests?

I'm interested in code written in C++.

Answer: 
The [Chromium](http://code.google.com/chromium/) project.
The [Bazaar](http://bazaar-vcs.org/) version control system is obsessive about automated testing. Unit testing, feature testing, integration testing, blackbox testing, test doubles testing, testing framework testing, parameterized test suites, platform-specific tests, etc.

This has all sorts of nice consequences for the project, but I must refrain here from undue VCS fanboying.

EDIT: oops, I did not notice you said you were interested in C++. Bazaar is a Python project.