Until Google comes with an Office web application here is a list of Ajax-based Office replacements:
Writely - "The Web Word Processor" (note that for creating documents, it uses an HTML editor and then converts to Word format)
FCKeditor is also an MS Word-like web app.
gOFFICE - "a browser-based online word processor and desktop publishing program"
Num Sum - web-based spreadsheets - except only the author of a spreadsheet can edit it.
Kiko - Online calendar solution powered by Ajax.
called S5 - web-based Powerpoint
Webnote - web-based version of Microsoft's OneNote
thinkfree - online Office suite
Openomy - online file-system