Philly Diner is this building's third attempt to establish a 24hour diner in the Penn area. Their prices are a little steep and their menu can be a little erratic, but nothing is better for a college student than a place they can eat at 4am without worry. They carry the usual diner foods (breakfast any time, burgers, etc) and they have some more unsual items (like steak, etc). They have lots of cakes always on display and coffee is always available to the sleep-deprived.
The waiters change almost on a weekley basis (they cant seem to keep their employees long) and their menu never changes. Their prices are a bit steep (almost $9 dollars for a burger) but again, they can afford to put those prices when they are one of the few places open at all hours of the night (aside from the local Freshgrocer and 24hour McDonalds).
If you are hungry and need a quick fix, go to Philly Diner but be prepared to pay the hefty price.