Campaign Cartographer - Campaign Cartographer is the leading map making software for games (RPGs, miniatures and wargaming).
Tabletop RPG Map editor 2 - RPG Map II is a “tabletop RPG” oriented map maker, created with the “keep it clean & simple” philosophy in mind.
PyMapper - Drag and drop tile based mapping program.
Dungeon Scrawl - A dungeon scrawling tool by ProbableTrain
Dungeon Tile Mapper - Dungeon Tile Mapper is a web based tile mapping tool that includes many official D&D tile sets.
Roll20 - Roll20 is a suite of easy-to-use digital tools that expand pen-and-paper gameplay.