The local Master Builders Assoc (NAHB sister or something) - I've probably got enough referral work through them to cover the cost, but just barely. Also, we had health insurance through them. They were very pro-city growth, not very pro-rural growth, so I got out for political reasons. The workers compensation thing is messed up here in Washington, so the turn around to see savings on it was something like 3 years....We didn't bother, as our employee load is typically only 1 to 3 guys.
I'm a member of the National Ground Water Association & Washington State Ground Water, and serve on the board of directors for the WSGWA. I also do quite a bit of lobbying work with the WSGWA, doing a lot of hand shaking with politicians at the capitol here in Washington State, etc.