This is already making it's way around the "blog-o-sphere", but I've got plenty of friends who are into web design but don't follow this stuff, so check it out. The Yahoo! UI Library is a collection of JavaScript widgets for things like calendars.

The Yahoo! Design Pattern Library is a collection of design patterns for web interfaces. Design patterns are a concept from "enterprise" programming, where best practice approaches to typical problems are described in a way that can be used by programmers using different languages. These patterns from Yahoo describe typical web UI patterns, such as paging multiple lines of search results.

There's nothing ground-breaking in the patterns, but it's useful to have these spelled out. It serves as a checklist, or a specification, if you find yourself having to implement one of these things. Even better, if you're asking someone else to implement some bit of web UI, giving them one of these patterns - modifying it if necessary -is a good way to make sure you're both clear on what you want it to do.