Listed from roughly lowest learning curve to highest
Google Spreadsheets
Google Drawings
There are two main ways I've seen this done.
1) Use a table or a grid in the background and draw on top of it
2) Upload a picture of the map, add players, and cover parts of the map you don't want revealed*
DungeonGrid
Pros:
- Can count distance for you for moving character
- Can add auras around characters
- Different boarders for different tokens
- Can auto-resize tokens
Cons:
- Very hard (impossible?) to have non-square tokens (and therefore non-rectangular objects as well) on the map
- NO documentation
Roll20
Pros:
- Lots of really nice features (fog-of-war, dynamic lighting, built in dice roller, macros, etc)
- Well documented
Cons:
- Not all the features may be available for free
- Some of the nice features may not be as helpful for PBP (as opposed to live, online gameplay)
*If you're running a module or AP, this is my favorite choice. Once you extract a map without all the secret doors and such in it (which admittedly, can be a bit tricky), it requires very little work. You basically just have to cover parts of the map you don't want revealed yet. You also get all that built in art and textures of the map, although admittedly, it can be rather pixelated depending on the resolution of the image.
