For language teachers, these can also be good places to make contact with other language teachers and discuss issues related to teaching. There are two major types of online chat sites. ESL Chat Central at Dave's ESL Cafe would be an example of such a site. The only activity at Chat Central is chatting. Such sites are very easy to use.
MOOs are text-based virtual worlds.
In addition to chatting, there are other activities. However, in order to chat, move around the site, etc. However, once you do, there is a greater variety of things to do. SchMOOze University is an example of such a site. It is intended for non-native English speakers and other interested in intercultural communication.