"Chosen is a JavaScript plugin that makes long, unwieldy select boxes much more user-friendly. It is currently available in both jQuery and Prototype flavors." Nice, elegant.

"A Rails form builder plugin with semantically rich and accessible markup." This looks like it could be really, really good; will be trying this out in anger shortly, I hope. (I hate Rails' form helpers with a passion).

Some really nice code to make decent sliders in jQuery without compromising the underlying HTML.

Modular form componentry; at a first glance, it looks good, but I want to go over it in a bit more detail...