Clubs and bars in the developed world are very similar to each other as are the type of situations you can expect to find there. Let's create a definitive list of great situational openers that work in any club or bar! (Yeah, I know I tricked you with the title of this thread but with your contributions we can make the content fit the title.
Sitting bar girl
Situation: you want to get a drink, bar is busy, girl is sitting at the bar.
Put your hand lightly on her arm, smile and say "Can you make some room so I can get some drinks for my friends? This bar is crazy."
She will if you are confident and have a great smile. Next you can start talking about how insane busy it is and transition to normal conversation.
Waiting bar girl
Situation: girl at the bar is waiting for her turn to order. You make sure you stand behind or preferably next to her.
Put your hand lightly on her arm, smile and say "They are not very eager to get us a drink around here now are they?". Wait for response if there is any and continue: "Can't you use your womanly charm to get his attention? I tried to charm him earlier this evening but that didn't seem to have the desired effect."
You can drag this out by suggesting that she leans forward or whatever. Just be playful with it and there should be an opening to move to some good conversation.
Situation: a group of people are dressed weird. (last night I ran into a group of very cute 21 year old girls dressed as native americans, feathers and all. Some girl turned 21 that day, so yeah, that made sense
"So what is THIS all about?". I act very surprised and playful. They know they look weird and are eager to let you know there is a reason for it.
Situation: you're in a bar where you know lots of university or college girls hang out.
"You look like you are [in college/attending university]?"
Let her respond and come back with "I [can see it/thought I could see it] by looking at your face. I know it's a bit judgmental but look around you and tell me that you don't see a lot of people you KNOW are not studying, right?"
She will look around and agree with you because every bar has loads of people that look a bit slow even if it's only from the amount of alcohol they have been consuming.
Situation: girl is playing with her phone or at least not actively speaking to anyone.
"Are you being bored? Look at this place that is ridiculous."
Don't be serious but make sure it's apparent that you are joking. Most of the time it opens them right up but sometimes I get a bitch-shield or otherwise unhelpful response after which I pretty much repeat what I said before in a bit more detail.
"I just thought you looked bored when there are all these people around which seemed kind of strange."
Now add your own situational openers and let's make sure we're all armed to the teeth with awesome openers for any club situation!