Shared editing is one of Google Docs' more useful features, but sometimes you want to get comments from people without actually changing your master document. Docs now supports a "comment only" editing mode which makes that possible.
To enable the option, click the Share button, add a user and then choose 'Click comment' as their editing option. It's also available for Google Apps users, with additional tweaks for editing by members of your company.
[via Google Docs Blog]