I use plenty of different anti-spam plugins for both sign-ups and forms, on many different WP installs, in order to find the ones that work best. Currently, it is THIS plugin, and I've switched most of my sites over to using it.
My only criticism is that the name should not imply just registrations. I almost didn't consider it for one of my sites until I looked more carefully at the install settings on a site I was already running with it.
I am very glad I did.
Now it's time to buy a beekeeping book. :^}