A must-see area for nightlife is just outside of Philadelphia itself, an area called Manayunk. Many years ago, Manayunk was a rough blue collar neighborhood (and some of it still is) but the city has been developing it in recent years. Check out Main Street, which is lined with cafe's, bars, restaurants, and countless stores. At night, Manayunk's bars fill with twenty and thity-somethings. Some favorite bars in the area are: The US Hotel - a nice pub but with a slightly refined atmosphere (good food and music as well); Manayunk Billiards - this place is really a dump, but its got character - a great place for down and dirty pool-playing!
Castle Roxx is a deceptively small looking building on the corner of Shurs Lane in Manyunk (one block up the hill from Main Street). It's a restaurant during the day and a popular bar in the evenings all week long. With a medieval theme, it's an interesting spot in eclectic Manyunk.
Drink specials go on all week and the food is good. The burgers and sandwhiches are excellent and go for about $7 each. There's a late-night menu with usual bar fare in case you get the munchies.
Pitchers is definitely bordering on a hole in the wall...except for the multiple plasmas that line the bar. It's not big, the food is decent but not amazing, and things get crowded late at night when there's a good game on or people just feel like dancing a little (usually in small, small groups) - it's far from being a club scene. The beer isn't overpriced and it's just a simple place to go to have some drinks, probably before heading to somewhere where people aren't making a fool of themselves if they decide to dance for a while.
Dress Code: If you have clothes on, chances are you'll be let in. I would say just no sweatpants or pajamas, but you never know.