If you're like most people, you've got more feeds in your news reader than you can keep up with. What's neat about this script is that it not only makes it easy to subscribe but also shows you if it's already in your subscription list. If it is, it's shown with a checkmark over the RSS icon.

My only objection is that it's not posted to UserScripts, a central repository for Greasemonkey scripts.

Smart Google Reader Subscribe Button is highly recommended.