RasMol is a computer program written for molecular graphics visualisation. It was originally developed by Roger Sayle . It is the simplest and most easy to use molecular imaging software available.
Historically, it was a very important tool for molecular biologists since the extremely optimized program allowed the software to run on (then) modestly powerfull personal computers. Before rasmol, visualization software ran on expensive and exclusive graphics workstations.
Rasmol is the only open source molecular visualization software available. Technically, it has been superceeded by other non-open source software (e.g. deepview ). Rasmol is notorious for an irritating language (for selecting certain protein chains, or changing colors etc), but despite this is beloved by most molecular biologists.
Protein Databank (PDB) files can be downloaded for visualization from the Research Collaboratory for Structural Bioinformatics (RCSB) bank. These have been uploaded by researches who have characterized the structure of molecules usually by X-ray crystallography or Nuclear Magnetic Resonance (NMR).
Links
FreeRasMol
Research Collaboratory for Structural Bioinformatics Protein data bank