The Chemical Probes Portal has been updated, the new site includes a lot more data about the existing probes, reviewer ratings and their comments.

A chemical probe is simply a reagent—a selective small-molecule modulator of a protein’s function—that allows the user to ask mechanistic and phenotypic questions about its molecular target in cell-based and/or animal studies. These are tools not drugs, they allow scientists to investigate the relationship between a molecular target and the broader biological consequences of modulating that target in cells or organisms. In general the focus is on specificity for the target rather than pharmacokinetics.