The BitMEX APIs are open and complete. Every function used by the BitMEX website is exposed via the API, allowing developers full control to build any kind of application on top of BitMEX.
This kind of access is unprecedented in cryptocurrency exchanges.
BitMEX is offering 6 months of free trading for publication of any new open source project that uses the BitMEX API. See this blog post for more details.
BitMEX implements the full BitcoinCharts API starting at https://www.bitmex.com/api/bitcoincharts. The BitcoinCharts API is a simple way to access trade and orderbook data in a format that is compatible with other exchanges.