Center City Happy Hour Specials
Well every Wednesday during the summer is Center City Summer Sips - a number of places around the city get involved - http://www.centercityphila.org/life/Sips.php - some places can get really crowded though.
I think Marathon (on the Square at least - 19th and spruce) has a good deal every day with $4 nachos or flat bread pizza, $2 lagers and i think $4 wines.
also check out this site: http://foobooz.com/2008/04/recession-...
while not necessarily only about happy hour - Foobooz compiled a great list of "deals" around the city for every day of the week, different meals, happy hours, etc.