Here is my entry for the 2009 WordPress MU Plugin Contest.
SI CAPTCHA Anti-Spam by Author: Mike Challis
Adds CAPTCHA anti-spam methods to WordPress on the comment form, registration form, login, or all. In order to post comments or regiser, users will have to type in the phrase shown on the image. This prevents spam from automated bots. Adds security. Works great with Akismet. Also is fully WPMU and BuddyPress compatible.
Version 2.0 (25 Sep 2009) This all new version 2.0 added full WPMU and BuddyPress compatibility. WPMU and BuddyPress users can now protect comment form, registration, and login from spam.
Supports WP, WPMU, BuddyPress 1.0.3 and 1.1, even works in mu-plugins directory.