The Rack, Boston
Update: The Rack, although still an option in the Virtual Tourist Nightlife drop down list, has gone out of business and the The Hard Rock Cafe has moved in.
Depending on when you go, The Rack is good for different things. During the day, they serve reasonably tasty food. Early in the evening, it becomes a popular place to play pool with over a dozen well-maintained tables. Later at night, The Rack becomes a 'see and be seen' place to drink, dance, and flirt.
Dress Code: The dress code will vary depending on when and why you visit The Rack. During the day and to play pool, the dress is quite casual. As the evening wears on and the scene shifts, the dress code becomes more clubby.
The Rack combines many things in the same place. It's a pool hall. It's a pizza restaurant (the pizza is surprisingly ok, and cheap). It's a bar. There's a dance floor. Most nights it has live music - definitely check out the guy who plays every Sunday, when the place tends to be dominated by out-of-towners and tourists.
It's also a total meatmarket on Thursday, Friday and Saturday. The atmosphere on these nights is completely different to during the week.
If you're a guy in town by yourself, check it out on any night of the week. I mean, on a Tuesday night it's going to be pretty quiet, but there'll be people there to talk to, for sure (although most of them will be guys). Open until 2am every day, though watch out for special events which sometimes mean it's not open to the general public.
Dress Code: Varies. No shorts, sandals, shirts without collars, or baseball hats. On Saturdays, no jeans either. Be warned - if you turn up wearing the above, they won't let you in, even if you're a girl, and therefore usually immune to dress restrictions.
The Rack is a high end billiards parlor and club. It also has entertainment and is a great spot to spend the night with a large group of friends.
Dress Code: dress nicely