On my campus their we're a couple of rivalries. I wouldnt necessarily say they are hard core, cuz no one goes instigating stuff.

Beta and ADPhi
Beta and Chi Psi
SAE and FIJI (big fight between them my soph. Had 6 police cars outside SAE's house)
Theta Chi and Theta Xi
Theta and Kappa
AGD and AXO (back in the day..mid 90's)
AGD and Gamma Phi (current)
PIKE and EVERY fraternity

Im sure there might be more too.