Ok, I'm also new here but am amazed to see nobody has mentioned Tiled.

The blurb Says
Tiled is a general purpose tile map editor, built to be flexible and easy to use. In addition to its own map format, Tiled supports read/write plugins for using it with proprietary map formats or formats used by other editors.
A list of games created using Tiled can be found HERE and include Platformers, RPGs (both Orthogonal & Isometric perspectives) & Shoot-em-ups.