http://www.apartmentcities.com/Philadelphia-Apartments/Center_City_Philadelphia_Apartments.asp
I remember using this a while back when me and my roommates were trying to get the hell off Broad street Junior year.
http://www.4wallsinphilly.com/centercity.htm
This is a decent one for looking at area apartments.
And craigslist, believe it or not, has a world of places for rent or people looking for roommates. Some are even affordable houses right on Fairmount Street and Spring Garden. Type in Center city apartments, or philadelphia apartments, a lot of stuff will come up.
My advice to somebody looking to shack up over here - find a roommate/roommates. It is that much sweeter paying 500 bucks in rent a month over the 800-1000 you're gonna be paying with a studio or one-bedroom apartment. There are a ton of people that need roommates, especially those listed in craigslist. Unfortunately that requires a lot of work on your part to find out if its a good environment while in school and if the people are normal individuals, which is never a given in this city.