Tiled is a general purpose tile map editor. It's built to be easy to use, yet it is capable of catering to a host of varying game engines, whether your game is an RPG, platformer, or Breakout clone. Tiled supports plugins to read and write map formats other than its own map format.
Tiled is a general purpose tile map editor. It has a very simple and easy to use interface, and is capable of outputting tile maps in a variety of formats, such as its own XML-based format, TMX, which can utilize gzip compression, or a Mappy compatible format (via a plugin). It is being developed in Java, so that it is immediately usable via the web, or to be downloaded on any Java capable platform.