I myself use Wordpress.
(Now don't you go 'bah! wordpress!? FEH!')
WP started out as a blogging tool but nowadays is on-par with many CMSs.
On the customization front: It supports themes. Your own, or others, or mods of others. It even has a theme editor where you can modify the css and php files of the current theme without having to DL, edit, and upload it all back.
(Be sure to hover your mouse over certain elements, as they have some pretty nice effects, achieved without the use of flash.)