Bad Behavior is a set of PHP scripts which prevents spambots from accessing your site by analyzing their actual HTTP requests and comparing them to profiles from known spambots. It goes far beyond User-Agent and Referer, however.
Signup allows users to sign up (or register, as in register for a class) for nodes of any type. Includes options for sending a notification email to a selected email address upon a new user signup (good for notifying event coordinators, etc.) and a confirmation email to users who sign up.