Triumph Brewing company is a great Princeton Hangout. Its a nice brew pub restaurant that offers live bands on select days. It can get crowded when there is live music. I've never been too impressed by their food. It's a better bar than restaurant.
Dress Code: Casual
This is a really nice, "swanked-out" brewery with some great beers and food. There's a $5 in the cover, unfortunately, but there is oftentimes live music. The ambience is really neat, as you have giant pots you can see through glass windows where the beer is made. 3 floors. Really nice place. Make sure that you look presentable, this isn't just some bar. :)
Dress Code: Look somewhat nice -- jeans are ok, though.