RCSB Protein Data Bank (PDB)
An Information Portal to Biological Macromolecular Structures
NCBI: Structure
"The Structure database or Molecular Modeling Database (MMDB) contains experimental data from crystallographic and NMR structure determinations. The data for MMDB are obtained from the Protein Data Bank (PDB). The NCBI has cross-linked structural data to bibliographic information, to the sequence databases, and to the NCBI taxonomy." The NCBI 3-dimensional viewer, Cn3D, can be used to visualize these molecular structures.
ExPASy UniProtKB
Three dimensional images of hundreds of proteins.