What is Dungeonworld?
Dungeonworld is a Play By (e)Mail run by MADHOUSE. They can probably explain the game far better than I, but, basically, you run one or more characters in a "swords and sorcery" environment. Your characters can range from classic wizard types and priests through human warriors and off into the more unusual character types such as alchemists or "oaken men". Other game positions, such as estates and traders are possible, and some players have ended up in positions of power - the current Regent is a player character, for example. Of course, he doesn't get as much freedom as he might like...
The world is populated by a mix of other adventurers, run by other players, and "non player characters", some friendly, most not. To say the world is huge is an understatement - there are dozens of multi level dungeons to explore, all embedded into the "overland" - the wilds and woods of Bereny.
The game also has a lively player forum for more in depth role playing of characters.