MySQL DBMS, probably the most common database system in use on the internet. All the sites I've worked on have run off it, and I've done MySQL for Developers training. Still, the reference manual on the website is always useful for checking the exact syntax of statements.
Drupal is a PHP based open-source Content Management System. It runs, among other things, MTV UK's site, the Australian Prime Minister's site, the Emmys site, and this site, mpettitt.me.uk. That's quite a range...
