Wonderdraft has the ability to add a hex grid to your map using the Overlay tools. You can choose the orientation, size, line width and color. You can't however get coordinates for your hexes. Coordinates are very handy if you are creating a gazetteer to describe where particular points of interest can be found on your map. Coordinates are also handy if you are doing a hex crawl style game.
You can download the PC/Mac/Linux version of mkhexgrid at the link above. mkhexgrid is a command line tool so you'll need to download it and unzip it to convenient location. To use it, you will need to open up a Command Prompt and change to the directory you've unzipped mkhexgrid into.
Example: Creating a 50 px grid
mkhexgrid has nearly 40 options that can be specified on the command line. Fortunately you only need to specify a handful to get a nicely numbered overlay that you can use in Wonderdraft or overlay it on an existing map image using a paint package like GIMP. I recommend saving your command line to a batch file or shell script to make it easy to edit/update as well as preserve for later.
mkhexgrid.exe --antialias --hex-height=50 --image-width=1000 --image-height=1000 --grid-grain=v --grid-thickness=2 --bg-opacity=127 --grid-color=000000 --coord-font="Calibri" --coord-size=10 --coord-distance=19 --coord-color=AAAAAA -o 50px1K.png
- antialias - this turns on anti-aliasing to make for a more pleasing image.
- hex-height=50 - Specify the height of the hex as 50 pixels. Because these are flat-topped hexes, this is face-to-face distance.
- image-width=1000 - Image width in pixels.
- image-height=1000 - Image height in pixels.
- grid-grain=v - Hex columns are vertical or flat side up.
- grid-thickness=2 - Make the hex lines 2 pixels.
- bg-opacity=127 - This sets the background color to transparent.
- grid-color=000000 - Sets grid color to black. This is the hexadecimal value for black.
- coord-font="Calibri" - You will need to specify a font available on your system.
- coord-size=10 - The size of the font in points not pixels.
- coord-distance=19 - The distance the coords are offset from the center of the hex. This value will depend on font chosen and font size.
- coord-color=AAAAAA - A light grey color in hexadecimal.
- -o 50px1K.png - Specify the output filename. This example uses 50px1K.png
The resulting grid overlay and how it looks brought into Wonderdraft and then exported.