Something very cool, not pun intended. It's the DIY HVAC Zoning Project on Sourceforge (via Slashdot).

Sourceforge is a repository of all sorts of open source software development projects. From what I've seen, the vast majority of projects are for and about computers.

There's lots of straightforward information about HVAC systems that is applicable even if you don't implement their plan. If you have a central system, it's worth a read for no other reason than to better communicate with contractors and service people.

From the Slashdot comments, other interesting projects/comments:

Greywater recylcing: save water flushing
Electrical device controls: