If you just want software to play against, without the client aspect, I'd recommend an older version of Sen:te's Goban (version 3.2). Unlike the latest version,

If you just want software to play against, without the client aspect, I'd recommend an older version of Sen:te's Goban (version 3.2). Unlike the latest version, it's free (the latest version is $10). It's quite old, but it seems to still work. It uses an older version of the Gnu GO engine. It used to be able to connect to the Pandas server. I don't know if that part still works. You should be able to find download links. If the Mac version of SmartGO has features similar to the Windows version, it should be excellent. But, they've been focused on iPhone/iPad apps so it hasn't come out yet.