There are a multitude of go servers on the internet. Here's a brief list of a few common ones.
By far the most popular English-speaking server, KGS is a real-time (that is, you play your game in one go) server with a lot of features, including audio lectures by pros, tournaments, and a nice teaching interface.
Most of our members play on KGS, and the society has its own room - go to Rooms, Room List, then Clubs, and we're the University of Warwick Go Society.
OGS is a popular English "play by email" server - it's not actually play by email, though. You log on, start a game, and play moves when you can over several weeks. It has a clever automatic move system, where you can put replies for your expectations of your opponent's moves, which means games run a little faster as some whole sequences play out instantly.
Of course, there are many other servers - IGS and DGS are other English real-time and non real-time servers, and there are many more Asian servers.