KGS is a go server written by wms. It is one of the most popular ones for english language players.
a number of incredibly annoying things about the KGS interface has caused me make the following pledge. http://fuseki.net/home/kgs-pledge.html However, it's unlikely that any of these will ever be implemented. So KGS will get more and more behind the times interface wise, and will eventually be replaced by something with a modern UI.
http://beijinggoclub.com/cgoban-h.jar
is a hacked version of the KGS client which enables a bunch of features. They were added by reverse engineering the obfuscated java code of the official client. They are mostly just really obvious things that would be simple to implement in normal code... but took a lot of work to do through the obfuscated code. As of today, it doesn't work because the server bumped versions. I do not think there was any actual protocol change - but the server is rejecting the version number reported by the client. (Jan 28 2012- got a new working version. Wow, I forgot how much I love using the mouse wheel to scroll through moves..)