Dave Winer writes today about making spreadsheets able to fetch data via web services calls. Very handy. I'd expect you can probably already do this on Excel using VBA, or on your office suite of choice, but it's a little harder when you don't control the software.
A co-worker from Humanware had a lot of great ideas along these lines a couple of years back. This was before anyone had done a spreadsheet on the web [that was the first great idea :)]. The concept there was to make it so web-spreadsheets were really databases, and you could make simple range queries over the web using some sort of REST syntax. Then someone could do a spreadsheet with some sort of primary data, like temperature readings from their home weather station, and someone else could do another spreadsheet which combined that and data from somewhere else, and so on. So it's somewhere in between Dave's idea and Pipes, but boiled down to numeric data.