iD is a JavaScript-based OpenStreetMap editor with MapCSS rendering.
iD is written with the Dojo framework.
It's intentionally simple. iD is not a 90% editor. It's not even a 70% editor. It should let you do the most basic tasks while not breaking other people's data. Nothing more.
Same goes for the code. So go easy on the abstraction. :)
Speaking of percentages, it's about 1% complete.
We're initially targeting WebKit-based browsers and Firefox, using SVG. IE and non-SVG can come later!
The licence of iD is WTFPL and contributions to 'trunk' should accord with this. This does of course allow you to dual-license.

Getting started

Unzip and start playing!
All the code is in js/iD.

How it works

The code works similarly to Potlatch 2, but with a bit less abstraction. So, we have:

Connection: stores, fetches and saves data. (iD/Connection.js)
Entity (Node, Way, Relation): the data objects. (iD/Entity.js)
EntityUI (NodeUI, WayUI): the rendered graphic elements. (iD/renderer/...)
Map: the displayed map on which EntityUIs are rendered. (iD/renderer/Map.js)
Controller: the heart of the app, which does its work via...
ControllerState: the current UI mode. ControllerStates decide what to do in response to mouse/keyboard events. (iD/controller/...)
UndoableAction: the code to actually change the data, as fired by ControllerStates. (iD/actions/...)

The UI is more modal than Potlatch 2. In particular there's a "draw shape" mode (the "Add road or shape" button) and an "edit object" mode. The directory structure of iD/controller reflects this.

Other relevant code includes the MapCSS parser in styleparser/ and custom widgets in ui/ .

Getting started

Most of the interesting code is in the ControllerStates, which live in iD/controller/. Each one corresponds to a UI mode (e.g. "drawing a way"). Its EntityMouseEvent method takes the user's mouse event (e.g. "clicked on a node"), carries out any actions, and returns the new ControllerState (which might just be 'this', i.e. carry on with the current state).

Coding tips

Scoping in JavaScript is famously broken: Dojo's lang.hitch method will save your life. Make sure you include dojo/_base/lang (in the 'declare' statement). Then, when you're passing an instance method as a function parameter, use lang.hitch(instance, instance.method) instead, and Dojo will magically set the right scope. You'll see lots of examples of this throughout the code.

Instance methods and variables always need to be accessed with 'this.'. This is a fairly frequent gotcha if you're coming from another language.

You'll find various notes and comments in the docs/ folder. Feel free to add to these.

More help: ping RichardF on IRC (irc.oftc.net, in #osm-dev or #osm), on the OSM mailing lists or at richard@systemeD.net.