Building Work Exchange like WWOOFING

Howdy,

I have a paper to write on Engineering and Sustainability and am interested in introducing a possible building work exchange similar to WWOOFING.
The idea would be for a host to have a building project and he/she/they would host volunteers to work. This way volunteers would gain experience building without paying for expensive workshops, and hosts would receive free labor.

Does anybody know of any such exchange on the web?

Any ideas as to how to make such a building exchange effective?

Any comments would greatly be appreciated,
Dillon