LeoTech

Base2 has just come out in beta on Google Code. It’s hosted there and can be included straight off the Google Code server.

I think it’s a really neat library because it basically fixes all browsers so that the built-in DOM, events, etc work the same way. Instead of providing an API of its own, it makes the existing API work consistently and reliably.

Base2 Features:
- A fast implementation of the Selectors API
- Fixes broken browser implementations of the DOM events module including document.createEvent(), dispatchEvent(), addEventListener(), etc
- Supports DOMContentLoaded
- Fixes getAttribute()/hasAttribute()/setAttribute() (Internet Explorer)
- Implements a few other useful DOM methods like getComputedStyle() and compareDocumentPosition()
- Supports a variety of browsers including ancient browsers like IE5.0 (Windows and Mac)

Dean Edwards has also done the excellent Packer Javascript minifier. It has probably the most in-depth support for obscure language features that simpler minifiers tend to mangle.