Google has started an "invitation only" beta of its new Spreadsheet AJAX application. Notable feature is sharing: is you can let others view the sheet as you edit it, and even invite others to edit it concurrently with you, as Writely did/will do for text documents. For info, images, and videos, see here and here.

I've seen people dismiss this approach as useless. It might not fully replace a client spreadsheet, but  sharing your spreadsheet rather than sending XLS files back and forth can change the way many people work for better. I've also seen Microsoft people claim it will only affect OpenOffice, as they are both toys and not-real-app-as-MS-ones. Wake up! MS Office now has two different competitors, which can start to shake its monopoly in a serious way, cutting MS revenues almost in half. My impression is that after many skirmishes this is the first true direct battle of the two giant IT companies. We'll see how it goes.