Well, considering you are asking about the most widely used program in use by games, not specifically mappers, It probably would be paint. Most dm's would just hand draw a quick sketch, or crank something out in paint instead of investing the time and (or) money into getting a better program.
Im thinking amongst individuals who take more pride in their maps, but only use a general purpose graphics utility, it's probably going to be photoshop. Yeah, I know, it's too expensive for common users, but about 95% of photoshop users are using a pirated copy.
When it comes to map specific programs, I've seen alot of CC2 users out there. It's been around for quite some time, while CC3 has been out for just a little over a year.