Drupal, easy to use. I'm currently looking into their mulit-site ability so I can run a bunch of sites on just one codebase.
Yea, THIS is one of the main advantages of Geeklog and Drupal, though both scripts handle it differently. Geeklog keeps them completely apart, Drupal links them as sub-communities. With Geeklog you can have many, many sites on 1 installation (=1 update, =1 plugin installation, =1 webspace).
joomla for more generic development. very stable and many useful extensions. drupals taxonomy is a killer feature though. awesome for certain applications.