There are several programs, for instance
for DOS machines,
KPT "Bryce" for Macintosh,
or the Fractint (DOS/Unix) "Plasma" fractal type, which can generate
and/or render height fields.
Utilities are available which can convert
between GIF, POT, TGA, MAT, and some other heightfield data formats.
Some of the textures available in POV-Ray and other raytracers can
create interesting heightfields as well, with some tweaking.
I have heard rumors that some popular raytracing packages
(such as Caligari trueSpace) will begin including terrain generation
options or accessories as standard equipment. Not being a user of
those packages, I can neither confirm nor deny that rumor.
Some pages that feature terrain generation software: