[ashedryden]

We use Open Atrium which is an install profile of Drupal. It's great because the feature set (by default) is very minimal, but it caneasily be extended with any of Drupal's modules, or you can create your own features. I love it because it's open source, runs on my servers, and is basic enough that when I give it to clients they aren't highly confused or need training to get started using it.