jQuery is a modern lightweight library which provides a lot of power to easily do advanced effects.
The fact that Web2Py includes this by default makes it an obvious choice, but it would likely have been chosen even without that.
There are a lot of plugins available:
SahanaPy currently uses:
- ClueTip (used for the form field 'Help' popup)
- ClockPick (used by Web2Py for Date input)
- FancyZoom (used by Web2Py's T2 module to display images)