GPUDirect and DirectGMA can transfer data through PCI-express by DMA.
This technology can realize high speed data transfer without transfer by CPU.
GPUDirect supported by NVIDIA, DirectGMA supported by AMD, those technologies are almost the same.
NVIDIA GPUDirect | NVIDIA Developer
FirePro DirectGMA Technical Overview - AMD
I think most interesting case is when using NIC on PCI-express.
A PC have two NIC interfaces(one IF is upstream / the other is downstream) and one graphics card
can some data stream gather, analysis and output realtime. It can use like a FPGA.