CSS Bloom - Great examples of web design
RoutingNavigator - A plugin for RoR that shows all routes at the bottom of all pages in development mode only.
AssetPackage - Wrap up all stylesheets and CSS into one file, making load times quicker for your users.
Accessible Image Replacement - For screen reader software.
Zooming both text and images - For sites where you want the users to be able to choose different font sizes and have both the text and images with text on them resize.
5 steps to build better websites