Previously, SABRE (http://wordpress.org/plugins/sabre/) was the best way to block spam registrations silently (no need for annoying CAPTCHAs).
However, that plugin has clearly been abandoned and has degraded in accuracy as WP progresses, but the plugin remains unchanged.
After scouring the plugin repo for a replacement, I've decided on Anti-Captcha. All the other spam plugins are waaay overkill.
In your testing, how accurate are the spam measures of your plugin, I assume you use it for your own WP setups?
So far, it's let an obvious spammer through, but can't be sure how well it's guarded against others spam attempts.
Also, are you planning on supporting this plugin indefinitely for improvements and compatibility?