Level Up 17: Free App/websites to play real players for Go/Igo/Weiqi/Baduk

About Go, from BoardGameGeek:

In Go, players alternately place stones on empty intersections of a 19x19 grid. The goal is to enclose territory behind stone perimeters and, secondarily, to tightly surround and capture enemy stones. After both players pass, they add up their territory and deduct the number of captives lost, higher score wins. The concept is simple yet the challenge can enrich a lifetime. In truth many lifetimes, for Go has been played for thousands of years. Known also as Weiqi, Igo, and Baduk, the game today has a worldwide following. (Small regional rule differences exist, but they seldom affect play.) So welcome to this masterpiece that is the game of Go, a race for the geographical control of an unclaimed land. Experience running battles and swift reversals, bold invasions and painful sacrifices, each sally, each setback playing out to the gentle tap of stone on wood.