I think this is the only “official” API that the game provides, and you need a key for it.
The stuff developed by simoyd and later taken over by another player (angellus I think) has been open sourced, AFAIK.
That’s what’s running the color scanner and other stuff I think. There is more that you can do with it but boundlexx makes most of what people want more accessible. Early on they locked down some of the stuff that people had found by packet-sniffing and other techniques, I’m not sure how possible it is to directly (http) access things like login server data any more.
That’s around the time we lost BoundlessNinja IIRC.
EDIT: To clarify if anyone notices or cares, the references from James to “Creative Mode” there are because that was, at the time, the term they used for an earlier local version which also allowed the use of the LUA interface for scripting and modding in game.