Waterloo Ice House is an excellent laid-back burger joint. They have a big variety that goes beyond your burgers too, if you're not in the mood.
I've had their tacos, their plates and sandwiches but their burgers is where it's at. They also have really good music every weekend.
Tip: Make sure to arrive early because tables fill up, specially with popular acts, and the parking is limited. If you're too early, swing by the Waterloo Records, just next door. They have excellent music variety.