The limits of Google Docs are:
- Every one who work on the document needs to have a Google account.
- IE 7 may not support all features of Google Docs.
- If one person is viewing the document, the editing by another person in the same document becomes slow.
- Upload a word document and images in it disappeared.
Found some interesting tools:
ShowDocument is a tool of web meeting and document sharing.
Codoxware allows simultaneous editing from multiple users in the same document consistently and as they are being typed.
Acrobat.com is a product of online conferencing offered by Adobe.
In addition, a list of collaborative real-time eidtors is available on Wikipedia.