XCrysDen

XCrysDen (X-Window Crystalline Structures and Densities) is a crystalline and molecular structure visualisation program, which displays isosurfaces and contours that can be superimposed on crystalline structures and interactively rotated and manipulated.

XCrySDen can run on most UNIX platforms, without any special hardware requirements and has been also ported to MAC OSX (requires X11) and Windows (requires CYGWIN).