This is the Perlin noise based terrain generator from Ian Parberry, "Tobler's First Law of Geography, Self Similarity, and Perlin Noise: A Large Scale Analysis of Gradient Distribution in Southern Utah with Application to Procedural Terrain Generation". You don't actually need to know much about the paper to try it out, but you at least need to know some of the concepts. It will generate a square cell of terrain elevations using Perlin noise with an exponentially distributed gradient magnitude and save it in a DEM file called output.asc. After that you are on your own, which you may find disconcerting. If I might make a small suggestion, Terragen 3 will do an excellent job of rendering your terrain, and it reads DEM files natively. You can download an only mildly crippled version for free from http://planetside.co.uk/products/terragen3, but I should in all good faith warn you that the process of learning its foibles is not for the faint of heart.

The program will prompt you for a random number seed (9999 is as good as any), number of octaves (try 8 if you are not sure), the height exponent mu (a number between 1.0 and 1.16-ish, try 1.002 if your imagination fails you), and an elevation cap in meters (somewhat tenuously related to the maximum height of the terrain, try 5000).