Hi
I've used JSCalendar with great success in several projects. However, inmy current project,
Seconded on JSCalendar. You can either type dates or click a button and have a very effective popup calendar widget. It's fast as well, not clunky like so many of these things!
I'm starting to move to Dojo as a general UIframework.
Dojo and Prototype both seem to be extremely popular. The Yahoo stuff also seems popular... I have no real opinion having only worked a little with Prototype...
Ed W