WGo.js contains two main components:
playyou can play move and create new position with rules applied.
WGo.js also comes with powerful go player, or more precisely sgf game viewer, which can be embedded into websites. This player is designed to be unlimitedly extendable.
All code is free, licensed under the MIT license.
WGo.js has repository on GitHub. There are recent versions of all files and you can also use it for reporting issues. If you like WGo, you can also contribute.