AJAX Office Tools
Although I don't believe that online AJAX office tools will ever replace Microsoft Office, some of the coolest web applications using AJAX are those trying to mimic Microsoft Office behavior.  Here are some examples of making office apps available through the web.

This is an AJAXified Word Processor.  I still haven't gotten around to testing it out because the last couple of times I was there, there wasn't an online demo.  I guess I should just download the damn thing and test it out.

This is a pretty slick spreadsheet app that could grow to be really cool.  It seemed a little unresponsive last time I visited it, but I gives me some great ideas for future AJAX applications.