MXR Fullbore Metal
ModTone Extreme Metal
ModTone High Gainer
or if you're looking for something interesting,
Electro-Harmonix Metal Muff
For me if i want a "metal-ish" sound i just on all my drives and overdrives at once.
Not sure about metalcore, but so far an overdrive + BOSS DS-1 sounds pretty good to me =)