JavaScript Workshop » Eloquent JavaScript

Marijn Haverbeke has put together an online book titled Eloquent JavaScript that not only has some nice content, but also incorporates a really cool integrated interface for editing and running the example programs.

I would categorize these lessons as beginner to advance so there is something for every level of developer.

Introduction
Basic JavaScript: values, variables, and control flow
Functions
Data structures: Objects and Arrays
Error Handling
Functional Programming
Searching
Object-oriented Programming
Modularity
Regular Expressions
Web programming: A crash course
The Document-Object Model
Browser Events
HTTP requests