I created a simple anti-spam plugin for WordPress—all it does is reject comments from unregistered users which have more than some number of links in them.
The default is 10 or more links causes an error, but this can be set by the administrator.
I already use the Bad Behavior plugin, which certainly catches a lot of spam comments (671 in the last week, apparently), but I’ve found that a lot of the remaining messages that get through to moderation have tons and tons of links in them. Whereas real commenters almost never submit that many links.
The plugin allows the error message to be defined, as well as the link threshold. It should be easy enough to install, just unzip it, drop it into the plugins directory for WordPress, and activate it from the plugins page. Then change the defaults on the “Max Links Anti-Spam” page.
I should really come up with a better name.