OGMO Editor is described as 'free, open source, project oriented level editor made for indie game developers by indie game developers' and is an app in the Development category. There are four alternatives to OGMO Editor for Windows, Mac and Linux. The best alternative is Tiled Map Editor, which is both free and Open Source. Other great apps like OGMO Editor are LDtk (Free, Open Source), PyMapper (Free, Open Source) and MapForge (Freemium).
Tiled is a general purpose tile map editor. It's built to be easy to use, yet flexible enough to work with varying game engines, whether your game is an RPG, platformer or Breakout clone. Tiled is free software and written in C++, using the Qt application framework.
Map-making software for creating tactical-scale battlemaps for RPGs (or even some boardgames), with a large library of ready-to-use mapping content Add-Ons (including 10 free ones!). Also works with your own artwork/bitmaps, via drag-and-drop.