The question says it all, we know most of the hosted web-based UI prototyping tools out there, but we would like to have ours hosted on our own internal servers, preferably with on-line multi-user collaboration functionality (i.e. users modifying the prototype, making comments, etc. in parallel).
Any suggestions will be appreciated.
Sketchflow (in the top-end version of Expression Blend from Microsoft) meets almost all your criteria. It ain't cheap, but it is very powerful.
The deployment package of a Sketchflow build can just be placed on an internal server (no IIS required to deploy).
Multiple users can overlay their comments and pen drawings over the top of the screens. Their feedback is packaged as a unit and sent back. All feedback can then be loaded back into the Expression Blend project and the feedback from 1 or more users viewed overlaid on the correct screens.
It does not meet your multiple-user authoring requirement though, but as they say "too many cooks...".