I2D vs STRING for protein-protein interactions
I just looked and it seems I2D use the PPI from 5 organisms to predict interactions in those organisms, which have not yet been experimentally demonstrated, so its interolog PPI prediction tool by design (http://genomebiology.com/content/pdf/gb-2007-8-5-r95.pdf), where as STRING can be used to see what proteins are known to interact with a given protein, and what evidence is present for a particular interaction, which you then get a (confidence like) score.