I'm always interested in Dungeon Revealer: it's free and open source, it's really simple because it has everything it needs to play: map with manual fog of war, token support, dice roller and chat. It's a self hosted web app, so no client to install. The only problem for me it's my ISP will not give me a public IP, and my players got used to the automation of Roll20 character sheet (although 5e it's easier than competitors like Pathfinder or older D&D).