Box.net with Google Docs
The newest entry into this field is a one-two punch from Box.net and Google. The process is somewhat complex to link your accounts from each service together, but once you do you can create Google Docs from inside Box that can be shared by others. The downside is that you lose real-time concurrent editing that is found on the other services, but the upside is that you can do some primitive collaboration for next to no cost. The interoperability is only one-way -- meaning a doc that is created on the Google side can't be seen by the Box users.
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