Depressing indeed. Are there any other open source projects that are API compatible with a proprietary product/service? Is it ethical? I feel like it is but would be interested to hear HN's opinion.
But that predates my existence :-)
WINE is a clone of the Windows API. ReactOS is another one.
Also, we should include implementations of software that encodes or decodes data formats, such as audio/video/image codecs. There are numerous examples of free implementations of software that works with proprietary formats, which are analogous to a proprietary API.