I've now uploaded the WESNet Designs ExtendedCheckBoxList control which allows the developer to add "All" and/or "None" checkboxes to the beginning or end of a CheckBoxList.  As expected, these special checkboxes either check all or clear all of the regular checkboxes and are updated as the user checks or unchecks the regular checkboxes.  As this is done client-side via Javascript, there are no annoying postbacks.

In addition, the developer can specify the maximum number of regular checkboxes that can be selected so that when this number is exceeded the control either stops accepting additional selections or pops up an alert box.

The developer can also give two or more ExtendedCheckBoxLists all with the same options a "groupname" in which case the user is blocked from selecting the same option in more than one "group".

The control including its source may be downloaded by registered users of this site free of charge by visiting our Downloads page.  There is a demonstration of the various features of this control on the Projects-->WebControls page.