I have been going to Carl's Tavern since I was a kid. Unlike the mullet, Carl's is a party in the front and business in the back.
They have a great selection of beers on tap (about twenty), and the bartenders and servers are always friendly.
One of the surprises that you will find at Carl's is that they have the best lobster bisque for a great price. Their burgers are also tasty and cheap. Carl's is known for their HUGE fish sandwich, and even if you have a huge appetite be prepared to take some of it home in a box.
Carl's also recently added a deck for additional seating during the warm months. (They have outdoor heaters for the chilly spring and autumn nights as well). This is fantastic, as there isn't much dinner seating inside and outdoor seating is a bit of a rarity in Pittsburgh.
Carl's is closed on Sunday and it can get crowded fast, but those are my only complaints.