[hyperpape]

www.gokgs.com is the server that's probably biggest in the English speaking world. The desktop client is Java based, there is an Android client, and we're waiting for an HTML client (it's in closed Alpha). There are several other servers that cater more to Asian players, but you can certainly play on them.

There is also online-go.com, which is web-based. It has real-time games, but the majority of games on the server are correspondance.

Resources: senseis.xmp.net (an old wiki that has lots of content, but few active contributors), and lifein19x19.com/forum, a forum dedicated to Go.

[mVChr]

I didn't know about the KGS HTML client. Either I should hurry up with my side-project or abandon it, as that's exactly what I'm building (but obviously without the already established KGS user base).

[hyperpape]

There are sporadic reports about development on the KGS account for Google+.

[RBerenguel]

Didn't know you were in HN :D Hi!