Azgaar’s Fantasy Map Generator is a free, open-source tool that lets you create randomly generated maps with one click. It’s easy to get started, playing around with options like turning on and off rivers, political boundaries, terrain, religions, populations, and on and on. Once you start digging into the rich sets of options, you’ll find a serious tool that you can use to customize as much as you like.
The generator will even let you download vector (SVG) files, so your map could be suitable for printing as large as you like. If you’ve got an eight-foot gaming table and a poster printer, you could theoretically fill your entire tabletop with a detailed hex map of your world.