I can't believe this thread has got this far and no one has suggested Google Docs.
http://docs.google.com
The nutshell:
Everything lives in the cloud, but you can backup locally into a convenient zip file with all docs.
Docs (Word import/export), Spreadsheet (Excel import/export), Drawings (Visio), and others.
Works with any PC w/IE, Firefox, Chrome, Safari, iPhone, Opera, iPad, even better on Android
Real-time multi-user collaboration
Secure as far as Google goes, and can be shared as read only or read/write.