Transparent hex overlay for maps?

[platimus]

I wouldn't mind hearing lars' answer but...

If you get paint.net (name of the program, not a website; it's basically MSPaint on steroids), you can install the HexGrid plugin (by BoltBait, I think). You can then open any image (map) and add a hex-grid to it. You'll probably want to create a new layer for it until you get sized and positioned just the way you want.

(I have bolded some suggested search terms for finding the downloads of this stuff.)