The magical L.A.M.P. :
Linux
Apache
Mozilla
Pxxx :
Perl
Python
Php
...but also :
RDBMS : PostGreSQL, MySQL, etc.
Java / J2EE solutions : JBoss, Jonas (ObjectWeb), and all the Apache Foundation projects, etc.
also in the O.S : OpenBSD, NetBSD, FreeBSD
Zope, PhpGroupware, etc.