I think the results are pretty good with Idealist. They encourage their
people to put that they saw the ad in Idealist, so we probably have a
better idea who comes from them, as opposed to from anywhere else. We almost always post with them. I find them better than nonprofitoyster,
for example, but we post there because we bought a multi-ad package with them.
I have had valuable success with idealist.org, so it would be a good choice.