I was pretty excited to see the ajaxcontroltoolkit and have tried to use it quite a bit.  Unfortunately, I have run into walls with real world implementations of these items.  Fortunately, the authors have made it open source so making changes, though difficult at times, is possible.  I have customized the autocomplete extender and am currently working on the masked edit extender.  I will post all code once I have updates.  I would be interested in others to work on some of these extenders, or at least post some wish list items as I make releases.  I will see as we go along, what is needed to post the items to codeplex so maybe they implement some of the customizations.