Tiled is an open source program that we contribute to every month to support the development team. Check out Tiled at mapeditor.org for more information. The program allows you to save your maps in jpg and png formats for easy export/import into VTT's like Roll20. Hop on over to Roll20.net as well to see all of the innovative tools the team there has created to your online virtual tabletop experience.
Create your world!
Happy Mapping!