Yes it's free.
What is FurMark?
FurMark is a very intensive OpenGL benchmark that uses fur rendering algorithms to measure the performance of the graphics card. Fur rendering is especially adapted to overheat the GPU and that's why FurMark is also a perfect stability and stress test tool (also called GPU burner) for the graphics card.
It's primarily a GPU stress testing program however I would advise against using it as it puts an unrealistic load on your GPU and, in some cases, has been known to cause damage to GPU's.
What is 3DMark?
3DMark is a computer benchmarking tool created and developed by Futuremark Corporation (formerly MadOnion.com and initially Futuremark) to determine the performance of a computer's 3D graphic rendering and CPU workload processing capabilities. Running 3DMark produces a 3DMark score, with higher numbers indicating better performance. The 3DMark measurement unit is intended to give a normalized mean for comparing different PC hardware configurations (mostly graphics processing units and central processing units), which proponents such as gamers and overclocking enthusiasts assert is indicative of end-user performance capabilities.
3DMark is primarily a benchmarking tool, used for testing the performance of your system as a whole. If you want to stress test your GPU, I would definitely recommend this over FurMark. It is a paid program but there is also a demo version. This puts similar load on your GPU to what you would get in actual gaming scenarios so is a much more realistic test.
Hope this helps.