Leela is a strong Go playing program combining advances in Go programming and further original research into a small, easy to use graphical interface.
It is freely available, making it an ideal introduction to the game.
Download Leela 0.6.2 (graphical interface, you probably want this one).
Download Leela 0.6.2 engine only (commandline/GTP engine).
Leela contains an AI technique modeled after the human visual system and brain, a deep stack of artificial neurons processing visual input with each layer combining the previous ones into higher level features (a so called DCNN, deep convolutional neural network). This "neural network" has been trained with more than 32 million positions from professional-level go games and taught to predict which moves a pro player would most likely consider. In 19x19 games the engine will query this deep "neural network" during the search to focus on the most critical variations. The result is a substantial raise in playing strength (about 6 stones), and a more human-like playstyle, while still allowing the engine to innovate of its own.
A GPU is typically much faster than a normal processor at doing the kind of calculations the Deep Learning algorithms need. For example, an AMD R9 390 (mid-range videocard) can find the worthwhile moves in a Go position about 20 times faster than an Intel Core i5-6600 (mid-range CPU). Leela makes use of this to explore variations deeper and in more detail. You can notice that in analysis mode the critical variations displayed by the engine are much longer when GPU Acceleration is used.
You need a video card with OpenCL 1.1 (or later) capability. This is supported on all recent AMD and NVIDIA cards. Make sure the latest drivers for the video card are installed. The detected video card can be seen in the Help->About menu.
This only makes sense if the built-in GPU is faster at doing the Deep Learning calculations than the CPU itself. Intel CPUs are typically very fast, with a weak built-in GPU, so this is not recommended. AMD APU's typically have a slow CPU but a fast GPU, so it would be worthwhile on those.
Playing a game, showing predicted territory.