I've been playing with a writable Web application called NumSum, a Web-based spreadsheet. Here's a little test spreadsheet I created. They have a "blog this" feature which creates an iFrame of the spreadsheet. Here's what it looks like:
Go ahead and change some cells and it will recalculate. Your changes won't be persistent. This is unlikely to put Office out of business anytime soon, but as a demo application, it shows just how far this might go. Doesn't feel like a Web-app, does it?
Tip of the hat to Tim Shadel for pointing these out to me.
I'll have to put this on my GTD maybe-someday list.
For a definitely-today GTD application, make sure to take a look at NextAction both for it's ajaxy slickness and novel offline usability.