Mipui map tool
So, when I was asking for advice on how to run a maze in a PbP campaign, someone linked their application, Mipui. It's a fairly simple thing, a mapping utility with real-time updates where the creator of the map gets a "secret" to append to the URL to edit it. That lets them do things like change the map on the fly, or make certain parts of the map hidden (including the ability to reveal all locations within sight of a given point). It's not perfect, but it's not a bad solution overall.
Protip: If you're having your players start at one end of the map, it might be worthwhile to make sure that's in the "north" position so that they don't have to scroll down at first.