(the old posts previously posted here also exist on the wordpress site, so no worries)
Habitat for Humanity - local and international
eMi (Engineering Ministries International) - offers pro bono design work
Local church bodies should also be a great source to find where you can best fit.
Get involved - Impact life
(Legally, I guess I should note that these organizations probably do not know that I reference them, but I plan to do/say nothing that would dishonor anyone.)