“A good program for playing go on Mac is Goban,” (Your Move/Readers Write: Mac Go? 3/25/2013) suggests Porter Howland. “It also works very well as a stand-alone .sgf reader, and I believe it can be used to play online. Goban and its underlying game engine are both open source and distributed freely under the GNU General Public License. Currently, the GNU Go engine is not the strongest; newer engines implement recently discovered algorithms that are more efficient. For example, the Many Faces of Go game engine by David Fotland.”
“For a real beginner, you can’t do much better than Anders Kierulf’s Go Kifu, for iPad (about $10),” writes David Erbach. “For desktop machines, Goban has the gnugo engine behind it, with a very nice interface. It’s plenty strong for a program, but doesn’t have Kifu’s tutorial mode, so it’s not quite as nice as a teaching tool.”
In addition to GNU Go, Ke Lu suggests PANDA-glGo; they’re both available on the IGS Pandanet site. Peter St. John flagged Wikipedia’s listof computer go playing programs and of course there’s always the AGA website’s go software page.