Dungeonographer Pro Version & Worlographer Battlemat License
[As of late 2018 this also includes a license that will unlock similar (but improved) battlemat (dungeon/building interiors/battlemat scale wilderness areas) mapping functionality in Worldographer. Buying this gives you two keys: one for the older Dungeonographer, and another for the new battlemat mapping in Worldographer. (In Worldographer you will be able to go to the File menu and select "Check/Update Battlemat License" to enter that code.) This isn't ready yet, but we expect it by the end of March 2019.]
The pro version of this easy to use Dungeonographer dungeon & building interior map-making program. The pro version allows you to customize icons, add notes to the map, expand an existing map with new rows or columns of squares, among other bonus features.