Google recently announced the launch of the Google Visualization API. Theoretically, one will be able to create visualizations to embed in Google Docs, although only Google Spreadsheets supports this functionality at the moment. (I assume this will allow them to compete more directly with Microsoft’s Excel, which has charting functionality.)
I assume this builds on their charting API, although the new vis API is Javascript-based, while the chart API is http/url-based.

The gallery is also little sparse right now, but David Hyunh, of MIT’s SIMILE project has already adapted the SIMILE Timeline code to use the Google Vis API.

(A brief warning if you are thinking of trying any of these yourself: Most of the comments left on the gallery pages are along the lines of “Lack of documentation makes this impossible to use!”, “The description of what this widget is supposed to do was clear enough, but the directions to make it work are incomprehensible. Url? Column?”, “How do you use this??”.)

Many of the sample gadgets in the Google Vis gallery are similar to those that one can create with ManyEyes. (ManyEyes is Java based and generates embeddable applets.) ManyEyes has a couple of diagram styles that Google Vis lacks, namely network diagrams (graphs) and treemaps.

? I think the two are equivalent, but it’s unclear from the documentation.

A couple of other useful/interesting visualizations from the Google Vis gallery include:

HeatMap

MotionChart

TimeSeries