PISA
Protein interfaces, surfaces and assemblies service PISA at European Bioinformatics Institute http://www.ebi.ac.uk/msd-srv/prot_int/pistart.html, authored by E. Krissinel and K. Henrick. Quoting from the PISA website: "PISA is an interactive tool for the exploration of macromolecular (protein, DNA/RNA and ligand) interfaces, prediction of probable quaternary structures (assemblies), database searches of structurally similar interfaces and assemblies, as well as searches on various assembly and PDB entry parameters."[1]