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: 
AFAIK C++ Boost libraries - <http://boost.org/> - have broadly covered code base, and a policy that every new piece of code must have unit tests with it. Might be worth checking.
The [Chromium](http://code.google.com/chromium/) project.