Terrain Software

Highly subjective list based on my experience with the various tools.

Terragen 2

TG2 is a node-based tool used mainly to render photo-realistic landscape views. With the correct setup (see Terragen Workflow) it can be used to render all the maps required for use in the Terrain Tools. It is extremely powerful, but requires a bit of a learning curve. Most terrain needs for a game can be realized without going into the node network too much, which makes it a lot easier to pick up. It's my personal favourite due to familarity and because it runs natively on OS X.

L3DT

A graphical wizard-based tool that guides you cleanly through the process of generating maps. Its unique feature is the design map, which allows you to specify the target landscape in a rough sketch, which is then used by the fractal generator as a basis. It seems to have a node network behind the scenes, but I've never used that. Great for coming up with a basic map, I very often use the design map feature to get a heightmap to my specifications and then continue work in TG2 (L3DT can export to Terragen's heightmap format). RoyS posted a Workflow for L3DT.

World Machine 2

A node-based tool that has a very good node network visualisation. It supports shapes and other editing functions. I've only used it a few times myself, so I don't really have much of an opinion.

GeoControl 2

The only tool that is controls-based, no node network in sight here. I only used the trial for a while. It seems powerful, especially because it has some features that all the other tools lack, like river generation.

Terrain Composer

The first generator tool for Unity. It looks very impressive, I haven't used it yet, so I don't know what exactly it does and doesn't aside from what you can see in the videos. Very powerful, also fairly complex.