Word Press

Open Source Weblog Engine

based on PHP and MySQL (does not even support other databases!)

http://wordpress.org/

quick list of plug-ins (alternative ways to use), including E Commerce http://www.incomediary.com/18-alternative-ways-to-use-wordpress/

built/run by AutoMattic.

Sept'2011: if self-hosting, make sure you turn on caching if you have any "worries" about getting high traffic