Can I implement google maps for one real estate company?
We will not charge our visitors for this service.

http://www.google.com/apis/maps/terms.html

Still can not believe this is for free....

It is indeed free if your pages are free to the public.

I wonder if, in the future, the underlying Google map might possibly show paid advertisements from your competitors.

<< Still can not believe this is for free.... >>

thats the main idea of Google.. mostly free :)

It's free because it's advertising their technology. Google Maps API is one of the easiest to use. Try using JSON instead of XML, though.