GnuGo is easy enough for beginners I think. I also used a bot called Aya when learning, but I don't know if that is available for OSX.
Regarding clients, there are a few options. CGoban2 is the only client you can use to access KGS, which is currently the biggest English-speaking server. You can also try online-go.com which is entirely browser-based so you don't need a client, and there's also IGS, a Japanese server which can be accessed through their own client called GoPanda2 or other clients like qGo2. There are also two servers called Tygem and WBaduk which are mostly Korean. They have proprietary clients you can run through Wine. I think they're supposed to be accessible through qGo2 as well, but I haven't been able to play through it, only observe others' games.
Regarding clients, there are a few options. CGoban2 is the only client you can use to access KGS, which is currently the biggest English-speaking server. You can also try online-go.com which is entirely browser-based so you don't need a client, and there's also IGS, a Japanese server which can be accessed through their own client called GoPanda2 or other clients like qGo2. There are also two servers called Tygem and WBaduk which are mostly Korean. They have proprietary clients you can run through Wine. I think they're supposed to be accessible through qGo2 as well, but I haven't been able to play through it, only observe others' games.
Also, if you're just learning, http://senseis.xmp.net/ is a great resource.