Benzamidine

Benzamidine is a reversible competitive inhibitor of trypsin, trypsin-like enzymes and serine proteases.

It is often used as a ligand in protein crystallography to prevent proteases from degrading a protein of interest; the triangular diamine group at the bottom gives it a very obvious 'stick-man' shape which shows up in difference density maps. The benzamidine moiety is also found in some pharmaceuticals, like dabigatran.