Jason provides the most likely candidate for what you need.
A few other options.
1. Create a dashboard view with filters that shows scheduled/assigned as well as completed activities.
2. The out of box Activities report does a pretty nice job of this (and it can be added to your dashboard if you want).
3. The Activity Feeds feature actually auto-posts scheduled and completed activities. This may be going away but will be replaced with Yammer which will essentially have the same features.
Hope some of these help!
Mike