There surely isn't a web developer who hasn't heard of jQuery. jQuery is Javascript library that simplifies effects, events and Ajax interactions.
Today jQuery 1.1.3 just got 800% faster and it's only 20kb. This release see's a number of bug fixes and handful of enhancements. At the time of writing it had over 1200 diggs. Certainly a library I will be looking to use again in the future in conjunction with my skin for subtext.
For more information about other Javascript libraries there is a good post over at sitepoint that contains most of the big contenders : What Ajax Library do you recommend.
