Patterns are optimal solutions to common problems. As common problems are tossed around a community and are resolved, common solutions often spontaneously emerge. Eventually, the best of these rise above the din and self-identify and become refined until they reach the status of a Design Pattern.
~ IAwiki.net

Given the now ubiquitous use of javascript libraries such as jquery, mootools and YUI for rich interactions across the web, design patterns are more important than ever. With numerous options of implementing functionality such as drag and drop, select a date or expanding and contracting a div, using documented design patterns make sure you don’t re-invent the wheel.

Useful design pattern references include:

• Yahoo Design Pattern Library
• Design Patterns on Flickr
• UI Pattern Library
• Pattern Tap
• UI-patterns
• Konigi
• Search design patterns
• Welie – patterns in interaction design

And for prototype tool of the moment – Axure (thanks for the recommendation Nic) – there’s a library of design and interaction patterns here and another one on Google Code here.