We use the Drupal Honeypot module on PortableApps.com and it works well. Mainly to block spammers from flooding comments and forum posts with bots. As one of the world's largest Drupal sites, we're under constant attack. We still use captchas for user registration, though, as we have a ton of accounts created by humans and then posted to by bots, which seems to be the m.o. of some of the larger spammers now.