Unfortunately the full web page with sample pictures, etc. was lost
in a disk crash and I haven't had time to regenerate it. MolViewer is a
program for visualizing and modifying small molecules under NeXTStep. It
is capable of constructing peptides from scratch (given a sequence). It
has many display options and can be used to rotate bonds, dihedrals, etc.
It will even do some simple energy minimization.
It can be downloaded here: