Impressive Open Source Projects

There are so many open source projects out there that you may be excused for gasping when reading the subject line.
Quite a few projects though make your eyes pop as they stand out in the crowd. There seem to be two kinds of projects; One where there are exceptional individuals championing the project (Linux, Boost) and another where projects originated as company internal and/or commercial projects or that companies are planning to make money out of them.

That's why many projects (all of those listed below) have full-time employees from various companies dedicated to the projects - i.e. there is a company making money out of it in the end (or hoping to make money :-)).

Impressive, beside being very useful products, is also that their software engineering practices are often way above their equivalents in the commercial world. I.e. they use careful release planning; track bugs meticulously and use regression test suites.

Here is my top six (in no particular order) of impressive projects:

Boost - very good C++ class libraries - make you wonder why you bother yourself sometimes :-)
Subversion - a version control system aiming to replace CVS
Linux kernel - no I do not use it - it is still impressive!
Mozilla - Mozilla browser - heaps of Netscape time and money spent here!
Eclipse - Eclipse IDE - heaps of IBM time and money spent here!
GNU Compiler Collection - The premier open source C++ (and other languages) compiler