I'm in the process of creating a map (that I actually plan on finishing) and I've come across the issue of having too much info to be clearly displayed on the one map. One option would be numerous maps however that just seems so cumbersome. Instead I was thinking of the old Atlases I used to look at back in my schooling days and they all utilised transparent overlays for various data-sets (political maps, etc).

So, does anybody know of any software (prefferably free) or file formats that will allow most people to easily view a map with multiple layers?