The YUI library from Yahoo sees another big release in its 2.4 release.
I'm looking forward to implementing this one on our sites. We have been using the calendar widget quite alot on our backend systems (building confference administration system requires quite alot of date entry stuff), but we have also implemented other parts.
For those who don't know YUI, or haven't looked at it, do it. The documentation of the YUI libraries is great; good api docs, nice cheatsheet pdfs and great videos with experts from Yahoo but also from outside Yahoo, like Peter Paul Koch (PPK).