TileEd is a 3D tile map editor/ placement tool. It helps speed up painting of game worlds when working with tiles or modular pieces that needs to align to a grid. It is for painting top-down/ isometric worlds rather than first person worlds consisting of modular pieces.
While TileEd's simple tiles and prop tools can be use with most tile and modular art packs there are also a growing list of art packs which supports the TileEd auto-tiles system. Have a look at this page to see what is available.
A custom palette which can dock in or outside the scene view for quick access to the tile and prop sets.
Auto Tiles speed up painting of terrain and interiors like caves, dungeons, and basements by auto placement of walls, corners and transitions. The auto-tiles supports various systems which makes it easy to sculpt a world in your choice of style.
Prop placement tool to ease placement and aligning none-tile objects to the tile grid.
Tile Layers, similar in concept to layers found in image editing tools, makes it easier to group tiles and props.