We have just released a new Segmentation Wizard, which allows you to quickly create, add to, or remove prospects from lists. You can slice and dice data based on fields, custom fields, and list membership. The engine used is significantly faster than automation rules and should be the method of choice for most list building actions.
Note that future releases will allow further on the fly segmenting by activities (prospect has viewed a landing page, completed a form, etc.) and segmentation rules to be rerun manually.