Download Squad posts that Bryce 5.5, a program that creates photo-realistic landscape renderings and animation with vegetation and realistic skies, is available as a free download (registration required to get a free serial number). While it lets you create your own topography, it also accepts DEMs as input for the landscapes. This is one version below the current 6.1 release, but is still feature-rich. Unfortunately, it also has a very idiosyncratic interface that takes a lot of getting used to. Here’s a review of the program from PC Pro, with some screen shots.
There are other free programs for realistic landscape rendering, with some limitations compared to Bryce 5.5 but also somewhat easier interfaces to navigate:
Genesis IV: Available in a freeware edition for personal use, as well as several paid versions.
The free program 3DEM can convert a number of DEM formats to Terragen’s native terrain format, and to formats compatible with Genesis IV.