SABRE (Simple Anti Bot Registration Engine)
http://wordpress.org/extend/plugins/sabre/
This plugin, though less popular, has more tools for detecting the ‘bots that post spam to your blog. There are a host of measures that are available in this plugin, things such as words and letters in an image, simple or complex math questions (that only humands can read, solve and answer, proving there is a human behind the post), and various checks such requiring users registering for the site to answer an e-mail by clicking on a link to complete registration.
Pros: Lots of tools to block spamming robots
Cons: Lots of things the user must do before they are allowed to post a comment.
Even the quiet option of checking for JavaScript can block some humans from posting (up to 2,000 people per day at my site).