You may be looking for Drupal. The core is extremely light and you could easily configure it to do just that. Drupal's done a great job of only adding the most necessary features. I'd like to see them pull Book + Forum and add CCK + Views to core, but I think that they still need to keep some obvious out-of-the-box functionality for new users.
I remember when I chose Drupal. I downloaded the code of five open source CMSs and finally decided to use Drupal, mostly because the code was the shortest and most legible. It has grown quite a bit since then, but I would still recommend it.
