Monty's Steakhouse at Topanga and Ventura has a killer happy hour. When we were last there their FREE mnchies included fried shrimp, meatballs, fried zucchini, fried stuffed mushrooms and a couple of other things I forget. They're not stingy and you can nurse a single drink until you're stuffed.
BJs on Canoga has good happy hours from 4 to 7 (1/2 price on all mini pizzas, $2 off appetizers)
Monty's: I've never been, but I heard they have free appetizers during happy hour
Acapulco: cheap drinks and free buffet style food, but the quality is not so good
Uwink: drinks are not too bad, but I'm not sure what their happy hour deals are
Sagebrush Cantina in Calabasas: i think they have early bird dinner where it is 25% off your bill if you arrive before 6:00 p.m.