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. :) )

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 this project. We eagerly accept pull requests.
See open issues in the issue tracker if you're looking for something to do
All the code is in js/iD.

To run the code locally, just fork this project and run it from a local webserver. With a Mac, you can enable Web Sharing and drop this in your website directory.

If you have Python handy, just cd into iD and run

 python -m SimpleHTTPServer

How it works

The code inherits many elements from the Potlatch 2 and JOSM editors.

We store technical notes in NOTES.md. The basic architecture is as follows:

Map: vector rendering, panning behaviors & zoom/center state
Tiles: receives map centers & draw tiles as a background for iD
Connection: requests /map data from osm, parses this data and delivers it to the Graph
Graph: stores a javascript object of id -> osm object
History: stores multiple graphs which represent undo states

Relationships:

Map has-a Tiles
Map has-a Connection
Map has-a History
History has-many Graphs

Coding tips

This project has a few basic guidelines for incoming code. The cardinal rules are:

Soft tabs only
Everything should pass jshint without warning
Never write bugs

(you can write bugs). If you need a style guide, AirBNB has a decent one.

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

License

iD is available under the WTFPL, though obviously, if you want to dual-license any contributions that's cool. It includes d3js, which BSD-licensed.