1. PDB (Protein Data Bank) is the single worldwide repository for the processing and distribution of 3D biological macromolecular structure data. Structure data from PDB can be searched at PDB site and viewed using RasMol, Chime, and their derivatives.

2. MMDB is a 3D structure database at NCBI. It contains a subset of PDB excluding theoretical models. Structure data can be searched at NCBI site and viewed using Cn3d.

For more information on how to visualize 3D structures, check the sections Visulization of 3D structures and Visulization of 3D structures on Mac.