Google Spreadsheets, probably the most impressive application from Google Docs, tests some interesting features that are currently experimental and limited to trusted testers.
One of the features lets you record macros for common tasks. Instead of repeatedly selecting the formatting for a cell or a group of cells, you can record the sequence of commands that perform the formatting and click on a button to use the macro.
Another feature, code-named "Sketchy", allows you to insert different drawings. Tony Ruscoe spotted some of the images that will be available in a library that resembles Microsoft Office's Clip Art.
Google Spreadsheets' code also references an option titled "Solver", which could be a tool for solving optimization problems, and an option to protect sheets and cell ranges.
Update: Tony points to a post from 2008 with more details about macros in Google Spreadsheets.