Tool for wireframe

What are folks using for wire framing ? I have been looking at a few options that can be shared and used across a team and i think the best i have come across is

What do other people prefer ? Are there open source licenses we should be pursuing ?

Whatever we decide, I think it will be nice to get some wireframing done by GCI students, for the radiology user-stories that have been written. May not be usable, since they will not have context, but might be too…