All great sources for non-profits
I find it very effective. We received a couple dozen good resumes.
Overall, I've gotten fairly decent resumes from the list and we have
interviewed a couple from the list.
The sites I've used are Opportunity NOC's and CraigsList and have good responses from both.
In short, a posting on Craig's list is free, so there is little risk, but to date, the posting/s have not generated impressive results. I have had valuable success with idealist.org, so that might be a much better choice for your non-profit clients.
I have been particularly successful in the Bay Area with Craig's List, which is free to non-profits, and very well used by employees. Also Opportunity Nocs has worked for us in the city.
Craig's list was terrible (Los Angeles part of site). We received resumes from all walks of life for all different jobs.