Turn based game servers
Currently there is a lot of fuss about Wordfeud, a turn based Scrabble(tm) like game, on the tablets and smartphones. Turn based games are already as old as there is connection over the mail between people. There are played hundreds, thousands of chess games by people sending each other a postcard with the next move. This is called "correspondence chess". When internet became more and more popular, people started to play through email. I, for example, started to play with the IECG. Which is nowadays defunct, because server based playing took over the world.