I use MapTool. It works great for updates, because you just move the PCs and critters, zoom to the level you want, then export as a .png file. Most of the time I load that .png file into PaintShopPro just to crop it a little. It works great.
Maptool is designed for the GM to create all the maps, then use Vent or Skype to get people together online and play in real time. I haven't tried that yet, but all the bits are there. I just use it for creating maps. All the tools needed are there, and more.
It uses png files with transparencies for the art objects. So, you can layer the objects to get the right display effect. There is also four layers to put objects. Background, hidden, objects, tokens. So, the GM can put an invisible creature and track it easily while the exported pic doesn't show the creature.
If I don't have an overhead pic for a creature, I can open the bestiary pdf, click, copy, paste into PSP, background erase, resize and save as .png in about 30 seconds. Then, I can import that into the map. Creatures are auto-sized to small/medium/large/huge/colossal as you choose. Or you can go native size and grab the corner to resize. .png files can be rotated. Shift rotate goes in 1 degree increments. Without holding the shift key, it is 90 degree rotation.
Size: It can handle any size map. I have the Forbidden Island all on one map. A1 to EK185 is a little far out, but that is the far corner of the map. Converted, that is 925 x 715 feet for the island. I can scroll in or out to any section. While it is possible to do a map of the eastern En continent at 5 pixels per mile, that map did make my machine grind quite hard.
Light sources can be defined per object. PCs sight can be defined, including darkvision. Walls can be made as vision blocking. If you turn on halo colors for PCs, you can get the vision capabilities of the PC in a pastel overlay color, showing just what it can and cannot see. Fog of war can be manual or automatic.
And the whole map/campaign is stored in one zip file so it can be emailed to someone else. All the necessary art objects are included, so they don't have to have the same base set of art as you do.
MapTool has a couple of stumbling blocks though.
a) All but the latest required Java 6. Well, Java 6 has been killed. You can't get it from Oracle any more. I still have it loaded, and call Java6 directly in the batch file that starts my MapTool. The latest version is supposed to work with Java 7, but I haven't updated to it yet.
b) It doesn't come with many graphics for creatures, buildings, or other artwork. It has a few generic pieces and points you to various web forums to go grab the rest. For those that want artwork for it, I have nearly 3 GB of artwork. So, I have many critters, tables, treasure, weapons, land textures, etc. Anybody wants it, I can give you a small or large subset of that artwork.
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The following example took probably 15 minutes to create, and takes less than a minute to update. It is an oval with a dark gray outer wall and light gray center. Then, fog of war clear the same oval. Use the pen to draw the dark gray wall segments. Then drag and drop the stairwells, doors, and items. I think there are about 30 separate items. The bed, the four posters around it, and the bedspread are all separate and layered. Add the PC tokens and critters. Adjust "front to back" layering for a few items and its ready. Zoom and export. Crop the export from the 16x9 of my monitor shape to the section needed. Publish. Voila.
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