"WAPUL is a part of a project I've been developing for creating libraries of user controls, pages and master pages (although currently no support for a master page using another master page). Its aim is to avoid bodging or extra leg work when distributing (i.e. just one .dll file)."
Sharing controls and pages is a feature many people are looking for. Graham has a clever solution using code, a post-build step, and the aspnet_merge tool.