iD is a JavaScript-based OpenStreetMap editor.
It's intentionally simple. This is not a 90% editor - 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.
The licence of iD is WTFPL, though obviously, if you want to dual-license any contributions that's cool.

Architecture

iD uses d3js for graphics & managing databindings to the map. There's a tiny tiled-map core, but the majority of the action is in dynamic rendering of the editable map data.
This project aims to create a usable object model of OpenStreetMap data in Javascript that can be transformed by actions and serialized back into changesets

Getting started

Fork it and start playing!
Read the live docs, generated from source every hour.
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 much 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.

As well as the live docs, you'll find various notes and comments in the docs/ folder. Feel free to add to these.

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

Come on in, the water's lovely. More help? Ping RichardF on IRC (irc.oftc.net, in #osm-dev or #osm), on the OSM mailing lists or at richard@systemeD.net.