Furmark for Linux is available, in the form of GpuTest 0.7.0.

If you don’t want to use the gui, there’s lots of (undocumented) options – use strings on the GpuTest executable.

For example:

./GpuTest /test=fur /width=1280 /height=1024 /benchmark /benchmark_duration_ms=5000 /no_scorebox /print_score

to run the test for 5 seconds and print the results to the terminal without waiting for the score box to pop open/need to be closed.

Running the software this way has the additional advantage that video modes can be used that aren’t supported by the GUI, like 1280×1024 on my old screens.

And here’s the scores for various video boards. Each of them was made with the same command line:

GpuTest /test=fur /width=1280 /height=1024 /benchmark /no_scorebox /print_score

Results:

GeForce GTX 980/PCIe/SSE2 6189 points, 103 FPS
Geforce GTX 660/PCIe/SSE2 2879 points, 47 FPS
Gallium 0.4 on AMD BARTS (this is the free driver on a Radeon HD 6870): 1874 points, 31 FPS
AMD Radeon HD 6800 Series (same HD 6870, proprietary driver): 2489 points, 41 FPS
GeForce GTX 460/PCIe/SSE2 1862 points, 31 FPS
Mesa DRI Intel(R) Haswell Desktop: 656 points, 10 FPS
Mesa DRI Intel(R) Ivybridge Desktop: 219 points, 3 FPS
Gallium 0.4 on NVC4 (free driver on GForce GTX 460 from above) 114 points, 1 FPS
Gallium 0.4 on AMD RS880 (free driver on a Radeon HD 4250): 70 points, 1 FPS

Of  course i didn’t expect great results with any of the onboard cards, but i found it astonishing how good the Haswell GPU was, and how much the RS880 sucked. By the way, the proprietary driver doesn’t seem to support the HD4250 at all, so i couldn’t use it to speed up the card.