Tiled is an open-source, completely free map editor. It requires a bit more learning than Dungeondraft, but it's extremely powerful. You'll need to find your own asset packs, but many free ones exist online.
: A powerful, free web-based tool for "old-school" style maps. Inkarnate (Free Version)
A single purchase of Dungeondraft provides you with a lifetime license, including all future updates.
The only official place to purchase Dungeondraft is through its website, which processes payments securely via Humble Bundle.
