Scott has mentioned this before, but I thought it would be worth a few words on it.
Project Butterfly is a new tech preview from Autodesk. It is basically a CAD application on the Web, no software or download is necessary. You can import drawings there, and then view and edit them.
The big highlight of Project Butterfly is its collaborative features. A user can easily start a co-edit session, where both sides can edit the drawing in real-time. This feature, and all other collaborative features work even if you invite people without a Butterfly account.
You can also start a design review, and the other side doesn't retain a copy of the file or anything like that. No duplicates, no more managing many files of different versions etc.
And the most basic of the features is the share feature, which simply enables a colleague or client of yours access your drawing and edit it (given permissions to do so).
This is the link: http://butterfly.autodesk.com/
And this is our blog: http://autodeskbutterfly.wordpress.com
Product Manager, Autodesk Project Butterfly