Furmark on Linux and Mac? Yeah:
GpuTest 0.2.0 is the first public version of a new cross-platform GPU stress test and benchmarking utility. GpuTest is based on OpenGL and is available for Windows (XP, Vista, 7 and 8), Linux and OS X. The current version is available in 64-bit only and I don’t know if a 32-bit version will see the light of day…

This first version of GpuTest comes with 3 tests (other tests will be added in next versions):
*a stress test based on FurMark (OpenGL 2.1 or 3.2)
*a tessellation test based on TessMark (OpenGL 4.0). The graphic load is equivalent to the extreme tessellation level (X32) of TessMark.
*GiMark: a geometry instancing test (OpenGL 3.3). By default, GiMark renders 100’000 instances of a mesh sphere made up of 128 triangles.

All tests can be launched on Windows and Linux. Regarding Linux, I developed and tested GpuTest under Linux Mint 13 64-bit. I didn’t tested it with other distributions. On OS X, only two tests are available: FurMark and GiMark. The test based on TessMark requires an OpenGL 4.0 and OSX 10.8 supports only OpenGL 3.2. But GiMark is an OpenGL 3.3 test, so why can we launch it on OSX? Actually OSX 10.8 supports a subset of OpenGL 3.3 features and geometry instancing is among these features.

On Windows and OSX, GpuTest comes with a graphical user interface (GUI) to launch the different tests. On Linux, a set of scripts is provided to launch the tests.
@geeks3d