The facility to create pivot tables has been added to Google spreadsheets and there's a short video to explain the idea and how to use it.

Google has added pivot tables  to Docs' spreadsheets.

Pivot tables provide an efficient way to summarise large data sets and obtain alternative perspectives on it. You take the row and column (2D data) and add more dimensions that you can "pivot" around and view in different ways.The Docs blog works through an example and the data is provided as a template so that you can easily experimennt with it. It also has this explanatory video which demonstates that pivot table creation is quick and easy.

Pivot tables created using Microsoft Excel can't be uploaded into Google Docs yet - but as they are quick to create from the grid data that can be imported that shouldn't present a problem.