http://www.compbio.dundee.ac.uk/www-pips/

http://mint.bio.uniroma2.it/mint/Welcome.do

http://www.hprd.org/

Database of Interacting Proteins

http://dip.doe-mbi.ucla.edu/dip/Main.cgi

BioGRID

Biological General Repository for Interaction Datasets

http://www.thebiogrid.org/

http://www.ebi.ac.uk/intact/main.xhtml

I2D

http://ophid.utoronto.ca/ophidv2.201/

To faciliate experimentation and integrated computational analysis with model organism PPI networks, we have integrated known, experimental and predicted PPIs for five model organisms and human in the I2D database.

STRING is a database of known and predicted protein interactions.
The interactions include direct (physical) and indirect (functional) associations; they are derived from four sources:

http://string.embl.de/

The MIPS Mammalian Protein-Protein Interaction Database

http://mips.helmholtz-muenchen.de/proj/ppi/