The SDK is jointly developed by Embecosm (toolchain) and Adapteva (everything else).
The component sources are spread across multiple repositories hosted on GitHub, issues should be logged using the relevant issue tracker and pull requests are invited.
To build the eSDK start with the epiphany-sdk repository and see the associated wiki.
The Epiphany toolchain is based on GCC, GDB and newlib, and includes a functional simulator for a single core.
For details of the history, features and performance etc. see the blog post by Dr Jeremy Bennett.
Driver, library and BSP etc.
For info on the Epiphany driver, loader, runtime library and BSPs see the blog post by Yaniv Sapir.