ONLINE GO SERVERS
The most popular go server in Europe and America, and in my opinion the best. Here there are many teachers that will teach for free, especially to beginners. On Kiseido there are many chat rooms, and it's possible to chat with others while watching games. There are usually 700-1300 people online.
IGS is a Japanese go server where there are usually about 1500-2500 people online. About 70% of these people are Japanese, while the rest are from various countries from all over the world. The chatting options aren't as great as on Kiseido with few people ever talking, and it's hard to find teachers. However, I have seen professionals play on IGS, which is rare on KGS(although KGS 9 dans are typically pros or have pro strength).
One thing that differentiates IGS from other servers is its unique half-stone system. Basically IGS ranks work like this: 10 kyu -> 10 kyu+ -> 9 kyu -> 9 kyu+ etc. A 10 kyu would play against a 10 kyu + with regular 1 stone handicap(as black against 0.5 komi), while the same 10 kyu would play against a 9 kyu as black taking 5.5 points reverse komi. If the 10 kyu plays against a 9 kyu + it's treated as 2 stone handicap, and against an 8 kyu it would be 2 stones with 5.5 point komi for black. Personally I don't like this system, but I know people that do. It's a bit weird getting used to, and taking 5.5 points as black is a huge advantage.
Tygem is the biggest Korean go server, and to play on Tygem you previously had to understand Korean, since the client didn't support other languages. Since march of 2011 however, Tygem has an English client. Tygem has some of the strongest players online, this is because Korea is known for having the strongest amateur players, many of which who can, and has beaten top pros. However, the Tygem strength level, at least in the dans, are about 2 stones weaker than KGS or IGS, so there's a high variance at 9 dan. Tygem has a lot of users, sometimes going over 30.000. Almost all of these people are either from South Korea, Japan or China. Even though 30k is by far the biggest pool of online go players available from a go client, the English speaking community just isn't there yet(They are playing on KGS).
Tygem is that it has a lot of aggressive players, as opposed to KGS which generally has more passive players. By playing on multiple servers one therefore gets to play against a wide variety of styles. If you are a beginner though, it may not be a good idea to play here, as there are a lot of sandbaggers in the beginning ranks(people that play on a level far beneath where they should). In order to download Tygem you must first register an account, and it's important not to use ad-block or the download link won't show up.
WBaduk(World Baduk, previously known as cyberoro) is a Korean go server that also has an English interface. WBaduk has a fairly high user pool, almost as big as tygem, with many Koreans and Chinese players. The rank-by-rank strength level is about the same as on Tygem as well. Like on Tygem, I've heard there's a lot of sanbaggers at the kyu level. I also remember this myself from the time I played on cyberoro when I was a beginner.
The nice thing about WBaduk is that it comes with about 2000 free life & death problems. Another thing I like about both Tygem and WBaduk is the voice countdown when in byo-yomi. The client is a bit buggy though. To log in you cannot simply select a server in the startup screen and log in, you have to select a server from the top-right "Server" menu first.
Fly or die is a website that has a lot of games, including go. On fly or die there are about 50 people playing go at any time. The playing level here isn't that high, with most players being around 30-10 kyu. There's usually one or two stronger kyu players on here, but it's rare to see mid or high-level dan players.
Until recently, there was only one board size to choose from, which was a 15*15 sized board. I've always found this to be a bit silly since 15*15 is quite unusual. That's not longer the case though, it's now possible to choose what board size you want to play on(9x9, 13x13, 15x15 or 19x19). The default time limit is 40 minutes for each player, but players almost always use around 5-10 minutes in their games. Unfortunately there's a lot of cheaters on Fly or Die, who will drag the time out, refuse counting and so on. Don't take this server seriously.
This is another popular place to play go. What makes this website different from the rest is the great amount of time given to the players(The default value is 90 days). You can also play many games at the same time. This makes DGS perfect for those that don't have time to sit through a whole game. Playing these types of games is also good if you want to deeply analyze board positions since it's possible to download an sgf of the game, and use your time figuring out where you want to play the next move.
Online Go Server is a turn based server like DGS, but also has regular live playing. It's popoular amongst beginners and double digit kyus, but don't expect to find to many live matches on the dan level. There are also some chat rooms, and about 40 people in the main one. OGS is unique in that it is a web based client. You play in the browser instead of having to download anything, and it has a fairly modern look.
One issue I've noticed with OGS is that the rankings seem a bit unreliable. People generally shy away from using handicap stones when ranks differ, and OGS also rank 9x9 and 13x13. There are seperate rankings for different settings, but they all affect your overall rank, and in my opinion that quickly gets confusing. You might have someone in the chat window ranked a certain rank, but that isn't the rank he will have when you play against him. What I like about OGS though is that it's a growing community with continuous updates to the website, so it has great potential. The userbase is smaller than KGS but catching up. Like KGS, most of the users are European or American.
Up until april 2014 you could also play go on yahoo!. There were usually 100-200 people playing, and it was hard to find games when you're new since most players wanted to play against people with established ranks. Also, there were many escapers(people who leave the game without resigning). Even though there were not that many players on yahoo!, the level of play was quite high.