For those who work with Geographic Information Systems (GIS), ESRI's ArcMap is the be all and end all. As one geographer put it "the beast lives off government and corporate contracts. It is able to charge a $500 licence for a single user. It is GIS." There are other systems out there, mostly freeware, but since most people are trained on ESRI software and professionally work on ESRI, the learning curve is step and many people shy away from other programs.
Christopher Brown, however, made a book about various other OnlineGIS programs. He reviews each program and gives his opinion on what each own would be a best fit for. He wrote a book, Online GIS, which is available for free download. (Hat tip: GISLounge)