Mac OS X only: Free Adium plug-in Challenge Response prevents instant messaging spam by requiring unknown users to answer a quick challenge question before interrupting you with their message. Challenges would look something like, "What is the square root of 49 in numerical form?" which requires a response of "7," but you can customise your own question and answer. Once a user answers correctly (i.e., verifies intelligence), Challenge/Response adds that user to a whitelist and they won't be challenged again. There's always the chance that the challenge and response might confuse an actual human enough that they'll just give up on IMing you, but if you're used to a lot of IM spam, Challenge/Response is a must. Challenge/Response is a free download, Mac OS X only, requires Adium. Thanks Jason!