plotting 3D electron density difference maps

Hi there,
 I am looking for software, preferably freeware, to plot 3D electron
 density or electron density difference maps for large molecules (appr.
 500 atoms). The program may run on a unix workstation or on pc.
 We have our own programs to calculate the value of the electron density
 (difference) in the grid points of a given 3D lattice. Therefore, we can
 generate the results in any format needed, but we do need to know what
 the input format is for a given plotting program, so therefore we would
 prefer a program that comes along with its source code or sample input
 files. We also welcome programs for converting one 3D data set into
 I already donwloaded Molden, but it seems a bit slow when I am looking
 at our molecular system interactively.
 Friendly greetings,
 Dr. Anik Peeters
 University of Antwerp
 anik $#at#$