There's so many to choose from, but no easy way to find what you need.
Here's some more examples I like.
Cufonized Fly-out Menu: Vertical menu with sliding hover highlight and supplemental fly out descriptions.
Star Rating: Rate something with (5) stars and half stars.
Lazy jQuery Plugin Loader: Load jQuery js files on demand
Inline Form Validation: Tooltips for confirmation or error messages
Auto Suggest: Auto-completion similar to many email clients
FullCalendar: A calendar similar to Google's that you can populate with events or even use with Google's data.