I love Go! I'm virtually obsessed with it. Here's all the info you need to start playing on your Mac. I've separated the list into two sections: servers and programs.
Servers:
Kiseido Go Server (KGS) is a modern online Go server. Kicks Yahoo's ass in every regard. Supports lots of advanced things like game reviewing and audio-based teaching. It's my personal favorite server. It uses a closed protocol, so there's only one client.
IGS is the granddaddy of online Go servers. Unlike KGS, IGS uses an open protocol. However, it's not as advanced. It's old as heck, and it shows. Even the documentation hasn't been updated in over a decade. The main advantage of IGS is the community: you'll generally find more strong players on IGS than KGS, and there's less inane chat. Also, IGS can be used on literally any computer with an Internet connection, because it was designed to be used with plain-text telnet clients (sometimes "old as heck" can be a good thing). Fortunately, there are many good clients available that shield you from the underlying text-based interface. Since the protocol is open, there are tons of different clients out there (see below)
Yahoo. I will not link, because Yahoo is evil. Use KGS. Better in every way. Even if my life depended on it, I would think twice about loading Yahoo Go again. It's REALLY bad.
Programs:
CGoban (for all major OSes) is the client for KGS. It's also a pretty solid SGF creator/editor.
Goban (for Mac OS X only) lets you play against the computer on any size board (using GnuGo as its core), and is also a very nice IGS client. The graphics are absolutely gorgeous. I highly recommend it.
glGo (for all major OSes) is another GnuGo/IGS client combo like Goban. the "gl" comes from its use of OpenGL to display 3D boards. Not bad, but I much prefer Goban.
<shameless-plug>
KGS Accountant (for Mac OS X and Windows) downloads and displays your KGS records and generates all kinds of statistics on them. It also lets you easily search your records and download games. And it was made by ME, so how can you resist? </shameless-plug>
Oh, and also, there are no really good Go programs for Mac OS 9. They're all ancient, and I can't find ANY of them online right now. However, I wrote an IGS client for OS 9 myself last year (got sick of not being able to play Go on my old Power Mac 9600, and I sure wasn't gonna use telnet). It's far from full-featured, but it works. PM or email me if you want a copy. I also have MacGo, Taikyoku Igo Goliath, Smart Go, GoServant, ichiGo (OS 9/X), and Komi on my HD, so I suppose I could send those to anyone who wants 'em as well.
Oh, and by the way, I go by the name Ceiter (4k) on KGS and Mikuro (1k) on IGS.
