The Petzl OK oval locking carabiner was designed specifically to be used with pulleys, but it is also a very good all purpose carabiner for anchor setups. Because of the oval shape, when using a pulley it ensures that the force on the pulley lines up correctly with the anchor point. This is great if you are using a Petzl Mini Traxion to self belay or using just about any pulley to haul stuff. At $16 it's not cheap. You can get a lot of carabiners that perform almost as well with a pulley for half the price. Even a carabiner like the Petzl Attache or Metolius Element will work well in most situations where you would use the OK but cost 20-40 percent less and are more versatile. That said, if you are using a pulley a lot, this is a good biner to go with.
Petzl updated the OK since we last tested it.