Mashi: One of the stated goals of HTML5, is to replace the Flash plug-in, the gold standard for making sprites and letters dance across the screen. This change is slowly coming as the gaming and presentation industries start to duplicate some of the sophisticated tools available. Mashi is an impressive example of how the sprites can be set in motion offering more than several dozen standard easing functions for moving sprites along a timeline.
MooTools: Like jQuery and YUI, MooTools offers nice, browser-independent shorthand for manipulating arrays, divs, spans. Its custom library construction tool lets you select the functions you want. Check some boxes and get an entirely optimized version of MooTools with the functions you need and none of the bloat you don't.
VideoForEverybody: This is like sticking a video tag into your HTML. The behavior of many supposedly standard-compliant browsers is different enough if you try to support them all. A number of good HTML5 libraries let you write standard HTML5 video tags that will be replaced with Flash or QuickTime if the browser isn't ready to handle your video. This tool offers one of the better libraries, filled with features that operate in the background to smooth delivery on older browsers like IE8.
JSCharts: Among the libraries aimed at chart building JSCharts allows you to create charts in different templates like bar charts, pie charts, or simple line graphs.
Flot: This produces graphical plots of arbitrary datasets on-the-fly client-side, and it includes interactive features like zooming and mouse tracking.
Protovis: Some libraries go further for data visualization. The collection of demos for Protovis and D3, a name in shorthand for Data Driven Documents show how sophisticated constructions like Voronoi diagrams and network graphs can illustrate more than the up and down of some value.
Simile Widgets: These offer a different collection of views for data visualization and are more focused on maps and timelines. Each of these data visualization projects illustrate how you can come up with good ways of turning data into pictures that can help you absorb large volumes of information quickly and efficiently.