Tiled – Best free, easy-to-use and highly flexible level editor

Tiled (full name: Tiled Map Editor) is currently the most popular, powerful and flexible tiled map/level editor for all types of 2D games, developed by Thorbjørn Lindeijer in 2008 (written in Java, therefore it's cross-platform). It is identified as an edge tool for 2D game developers to quickly create game maps.