Jeremy Zawodny works for Yahoo! and knows MySql inside-out. He's speaking about open source and Yahoo!

There are several reasons Yahoo! uses open source:

• Flexibility - Yahoo! customizes lots of OSS for its needs
• Documentation is better in open source software.
• Availbility for the platforms that Yahoo! cares about
• Support is good and getting better.
• Cost is an issue, especially at Yahoo! scales.

Jeremy lists out many of the open source products in use at Yahoo!:

On the server side, Yahoo! uses FreeBSD/Linux, Apache, C++ (and GNU tool chain), PHP, APC (caching and acceleration), Perl, and mdbm/MySql.

On the development side: Bugzilla, CVS, Request Tracker, Valgrind, Emacs and VIM, gcc/gdb, PhpMyAdmin.

Others include Python, Ruby, rsync, BIND, Qmail, Squid, ImageMagick, SSH, zlib/gzip, NNagios, rrdtool, Boost (C++ libs), Many CPAN modules, PEAR, and many more.

Yahoo! is also working on opening up APIs including RSS feeds, Flickr, and Konfabulator (Yahoo! Widgets), and the Yahoo! development network.