Via reddit comes a pointer to the Lucid Hydra 200: Vendor Agnostic Multi-GPU. From the article:

At a high level what Lucid's technology does is intercept OpenGL/DirectX commands from the CPU to the GPU and load balance them across any number of GPUs. The final buffers are read back by the Lucid chip and sent to primary GPU for display.

Obviously more interesting for serious gamers, but still very cool technology, if it works as advertised.