Alternate Terrain Generation (ATG) is a complete replacement for the vanilla overworld terrain generator which radically changes the way the world looks, and the way in which you will play. It was originally created for Minecraft 1.5 to address issues with the juxtaposition of hot and cold biomes, and to introduce more realism into the way the world was generated as a whole.
Rather than picking a biome for a region and then building the terrain for that biome, ATG generates a series of fields for height, temperature, rainfall and more, in order to derive the final biome from these. The result is a smoother variation in the terrain, and biomes which would realistically be found adjacent to one another.
ATG also adds a noticeable degree of verticality to the world, and you may find flat plateaus on the sides of rolling hills as high as 150 or more, with the biggest of the snow-capped mountains reaching 240 or more.
Version 2.0.0 is a complete rewrite, making full use of new techniques that I have learned over the last few years. There's a lot more flat land, islands, cliffs, better biome distribution, while still having the trademark height variance of the older versions (though more spaced out in general.)
Additionally, due to a total rework of the internal framework for generation, it's fast. Really fast. The biome generation stage of generation, which was the slowest part in previous versions is now over 8 times faster than vanilla. This doesn't translate to 8x faster chunk output, but it's a noticeable difference.
How to use ATG:
When starting a new world, select the "ATG - Alternate" world type in the settings.
For dedicated servers, set the "level-type" in server.properties to ATG. You'll need to start a new world after that, because the level-type is only considered when creating an entirely new world save folder.
Configuration and Mod support:
As of ATG 2.0.0, there is no configuration at all. This is not always going to be the case, and is simply because getting the rewrite out there with a basic feature set is currently very important, and the configuration and customisation code has yet to be written.
Eventually it will be possible to set everything up per world via the customise world menu at world creation, with presets saved and exportable as json for the server generator options field.
Biome mod compatibility is currently non-existent. Again, this is just because the code for setting it up isn't there yet. The system will accept any biome and work without incident, there's just no way to tell the generator to use a new biome yet. All other aspects of generation besides new biomes should be compatible.
I'd like to give a massive shout out to ScottKillen, who helped me during development with advice and some excellent examples of the Forge terrain generation hooks that he added. He helped me to understand the particulars of the vanilla generator and how best to change things.