WP-SpamFree
http://wordpress.org/extend/plugins/wp-spamfree/
By far, this is the most popular plugin to date. WP SpamFree works by changing the comments form so that it uses JavaScript and Cookies. It blocks any comment that comes from a host that doesn’t support JavaScript and Cookies. The author does this on the basis that most comment spam is posted by ‘bots which don’t support JavaScript and Cookies.
Pros: Very simple, invisible to the posters.
Cons: A significant number of humans are blocked from posting because they disable JavaScript or run a program that disables it selectively such as NoScript for Mozilla Firefox.
There are a significant number of people that turn off JavaScript to block advertising and malicious attacks, or run Mozilla with the NoScript plugin for the same reason. These folks constitute roughly 5% of the visitors to my site, which is more than 58,000 people a month and up to 2,000 people per day.
That’s a LOT of people blocked because of they disabled JavaScript!