Writely is a web-based word processor and collaboration application. I"ve been using it all day with another author to collaborate on a magazine piece. It works excellently in Firefox on Windows and with just a bit of clunkiness in either FF or Konqueror on Linux, though it is certainly usable on the alternative platform.
If the pundits are to be believed, and I don't see why they shouldn't be, web-based apps are the shape of things to come. Here's a chance to dip your toe into the water, albeit in a small way.
I am not yet ready to give up my Open Office installations. OO 2.0 is my preferred office suite now and I have not yet missed MS Office.
On a related note, there is a portable version of Open Office that can be carried around on a USB pen drive. You can see it here.