If you want a collaborative spreadsheet, you could try NumSum. Or, you could download and install the alpha of WikiCalc, which was created by none other than spreadsheet pioneer Dan Bricklin, who brought us VisiCalc back when dinosaurs walked the earth. Right now, the only version of WikiCalc is a Windows desktop app, but Bricklin plans on making it cross-platform, and producing both client and server versions, so it shouldn't be long before you can use this on the web. He's also planning to GPL it, so if you want to help out, here's your chance to work with code created by a software legend.

[Via the Office Weblog]