You can create quick-and-dirty relief maps, with user-defined parameters for land to water ratio, number of towns, and map colours. You can define the pixel dimensions of the map (the default is 640x480, but I like 1024x768; for simplicity, I recommend a scale of 1 pixel = 1 mile). You can also save your custom parameters to a profile, which you can reload the next time you run the tool. As an added bonus, you can export the results as bitmaps.
The town name database is essentially a plain text file of names. I use EBoN to create random names in a language I want, then cut-and-paste the results into their own text file. Greenfish lets you assign the town DB you want to use, so you can vary the language of town names from map to map.
On the downside, Greenfish does not draw rivers (though it will create lakes). Also, landmasses often continue "off-map," meaning that the tool does a good job of creating sub-continents, but not so well with continents or what we might consider a "global" view.
That said, this tool is great for creating campaign areas or "regional maps" quickly and without the fuss of generating an entire game world. If you're industrious, you could create several maps and "stitch" them together with an image editing software. This requires some time and effort, but if you start with a bunch of small maps, you can create fairly compelling results.
Use this tool if you just want a campaign area, as opposed to an entire global map. It's fast, it's customisable, and it's free.