DMM Revolver

The Revolver deserves to be a lot more popular. Its ostensible exorbitant price is misleading as the 'biner will almost certainly pay for itself in extended rope life if you do much toproping and/or lowering.

What it is:
A carabiner with a rolling bearing in the basket. This bearing greatly reduces biner/rope friction.

Advantageous uses:

Top roping and lowering. I haven't seen any data on this issue, but my gut feeling is that regular use of the Revolver in this capacity would at least double the life of a rope which is subjected to a lot of this one-pitch action.

Clip in point for directional pieces. Allows use of a short runner on a directional.

Clip in point for high rope drag potential pieces such as at the start of a traverse or downclimb.

Clip in point for a the "fall piece" when working a route. Pulling oneself back up to the high point is much easier when the pulley point is a Revolver.

Doubles as a pulley in those situations where one would be handy -- hauling, crevasse rescue.

Disadvantages:
Expensive (but you probably don't need more than one or two on your rack) and relatively heavy. Also, it so reduces systemic friction that a light belayer could have trouble controlling a heavy climber; in this situation either anchor the belayer or don't use a Revolver.

The side of the basket, immediately adjacent to the bearing seems to wear more quickly than you would want it to. This is somewhat mitigated by the fact the the 'biner is thicker at this point.

DMM is set to release locking Revolvers which will be even more expensive and heavier, but that much safer, esp. for topropng.