Valve has made the Direct3D to OpenGL translation layer of Dota 2 available to the open source community. This piece of software enables the game publisher to take a standard DirectX Windows game and port it to Mac OS X or Linux/SteamOS. You can find the project at ToGL, with some tweaks it should be possible to make it work with other DirectX-based game engines as well.
ToGL intercepts calls to Direct3D and replaces them with an OpenGL equivalent. There's a performance hit of course, but it's relatively small. Unfortunately, ToGL only supports Direct3D 9.0c so this piece of software will probably primarily be used to re-release older titles on OS X or Linux.
The code, aptly named ToGL, was uploaded to GitHub by Valve developer Pierre-Loup A. Griffais — a fantastic name that he sadly abbreviates to Plagman. ToGL is taken straight from the Dota 2 source tree and supports a subset of Direct3D 9.0c, bytecode-level HLSL to GLSL (shader) translation, and some Shader Model 3 (SM3) support. The code is provided as-is and completely unsupported; Valve says you’re free to use it however you wish, and you can submit modifications to the GitHub repository if you like.