A provider of free online chess. Anyone can play with any compatible chess client. The chess.net people provide access to their servers via telnet (which may be used in conjunction with various chess clients for Unix, etc.). They also provide Windows clients of their own. chess.net for Windows Lite is a gratis client, but it doesn't have some of the useful features that chess.net for Windows has, such as the ability to save games and then recall them and play out alternate scenarios. The chess.net site also has a Java applet that you can use to access their servers.
The ratings for each of the types of play (Lightning, Blitz, Standard, Wild, Bughouse, and Crazyhouse) are split up so a users rating for each of the types of play is different, depending on how good they are at that particular type of play.
One of the especially good things about the place is that they have users of myriad different skill levels that you can play against, all the way up to even some GMs and IMs (which often have scheduled matches so you can plan to watch a particular match between some high-level GMs).
While there are usually only around 500 hundred users on at any given time, which some might argue is a disadvantage, it's plenty of people to play against at numerous different skill levels. While other services, such as Yahoo!, might have more players (which really isn't a great advantage anyway), those players are usually at a much lower skill level than one can find at chess.net. Many other chess services, such as Yahoo!, have no admins, no decency, and no protections against cheating.