Ghost Bar is located in the Palms casino and is unique in that it is not only on the 55th floor with an open balcony (there are several such clubs in Vegas), but that it also has a large square glass block in the floor of the patio that extends over the side of the building so you can stand on it and look down through the floor at the parking lot below. Well I should say, most of the time you can do that. On Friday and Saturday nights though, there's often a DJ set up in that area so you may not always have access, so go on a week night if you really want to see through the floor.
The wait to get in can be painfully long unless you have a casino host there, a huge group of girls or some type of VIP service. Table reservations do get immediate entry, however a table at Ghost typically requires a $400+ bottle and only gets you two seats (as of summer 2011), you need a second bottle for two more seats.
Aside from those gripes, it is still one of my favorites though thanks to the music and view. Ghost's music is typically EDM type stuff, whereas the other club Moon in the adjacent tower is more mainstream, top 20, hip hop, etc.
The other nice thing is that if you pay to get in one of the Palms’ clubs, you can get in all the others, including the Playboy Club and Moon which is also on the 55th floor of a different building with an open roof and two balconies.
For VIP access options; see here.
Here's the view from Ghost's north-side outside bar in March 2012: