The Unity Editor has been given a Linux port. The build is experimental and officially requires a 64-bit version of Ubuntu 12.04 or newer, but should still run on most modern Linux installations. A recent Intel, Nvidia or AMD GPU is also required.
The Unity Editor build for Linux is based on Unity 5.1.0f3 and supports exporting runtimes to Linux, Mac, Windows Standalone, WebGL, WebPlayer, Android, Tizen, and SamsungTV. Unity’s Na’Tosha states that future support is not yet guaranteed, and that “your usage and feedback for us will determine whether this version is one that is viable alongside our Mac and Windows versions.”
Compared to OS X and Windows, Linux has fewer game development tools. Epic Games does offer a Linux version of the Unreal Engine 4 Editor these days, and Steam does the same with its Source SDK. Stencyl and Ogre also have Linux versions of their SDKs.