Describe Unit of Measurement here.

Summarize

http://groups.google.com/group/sage-devel/browse_thread/thread/8791448b7a303ce9/807189c979add549

http://groups.google.com/group/sage-devel/browse_thread/thread/ae18ce618abd66d2/b440d40dd28b8ef2

http://groups.google.com/group/sage-support/browse_thread/thread/842c647087271b71/6b398b1f4309bd2e

etc.

What packages are out there? Pros/cons of each (social as well as technical aspects of the project).