Terragen is a landscape / scenery renderer. Its interface is not at all complex, and is actually rather easy to use. Even though it is easy to use, Terragen has produced some fantastic, photorealistic results, though, I can't lay claim to many of those. Nonetheless, Terragen is fun to have, and if you aren't I suggest you sign up to the Terragen mailing list.
A couple of useful Terragen links:
The official Terragen website - Planetside
The official Terragen mailing list - Terragen ML
Terragen resource site - Terrasource.net
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