Arsen Lüpen lies on Sakizaðacý Sokak, off Istiklal Caddesi in Taksim. To get there walk down Istiklal Caddesi until you get to the Aða Mosque on the right. At the mosque turn right and walk to the second right where you will see a sign on the corner that says "Arsen Lüpen" the entrance is around the corner on the right. Walk up to the third floor where there is a terrace outside. Arsen Lüpen is a small place where a lot of the alternative crowd of Istanbul likes to hang out, discuss Marx, and show off their latest Guevara t-shirt. Even if your politics aren't left, nobody will mind, and this is just a great place to kick back, grab an Efes, listen to alt rock, and spend a warm Istanbul summer night. The drinks are cheap, and the people are nice. A word of warning, the waiters don't accept tips, a friend of mine made that mistake once, and the waiter kindly told him that he chooses not to accept tips.
So whether you are just looking for a relaxed evening at a chill bar, or want an intellectual debate on the prospects of socialism in Turkey, check out Arsen Lüpen.