It's Google Docs
So the question is, is Google Docs now so firmly embedded in my working practices that I couldn't live without it?
No. If it went away (or started charging more than a nominal fee), we'd simply move on a congregate round one of the other web office suites that are out there (and don't forget, Microsloth Office online, coming to an everywhere near you, soon).
Redundancy - essential (in this context).