WP-Deadbolt is a plugin for WordPress 2.1
In addition to the plugin, I have provided an alternate release that allows WordPress 2.0.x users to achieve the same result, however, doing so requires that you edit one core WordPress file, wp-register.php. The need for this stemmed from the lack of a hook in WordPress 2.0.6 and below.
WP-Deadbolt (the plugin) and deadbolt (the alternate release) provide a means for WordPress blog admins to restrict certain e-mails from being registered.
For instance, do you get a lot of signups from a domain that appears to be spammy? Now you can block them from registering with that e-mail address.
Don't like gmail, or hotmail? Block em with a deadbolt. Done deal.
Details on both files, and downloads are available at http://www.village-idiot.org/archives/2007/01/10/wp-deadbolt/
PS: If the neccessary hook that is provided in 2.1 is later added to the 2.0.x series, the 2.1 plugin version will work. At this time, as of 2.0.6 it does not.