In reply to alan moore:
For the few pennies extra which it costs I prefer to have a dedicated belay device for half ropes, and for that my favourite is the ATC Alpine Guide (the green one). I don't like being belayed on 8 mm or 8.5 mm half ropes in a high fall factor situation from a full size ATC or Reverso, especially if clipping the two strands separately.
For the wall and sport climbing (which I realise you didn't ask about ) then a Grigri or an Edelrid Jul. I have switched more or less completely to the Grigri now, but one feature I like about the Jul is that it is made from stainless steel and so doesn't wear thin from the frequent lowering off in the same way as an ATC or a Reverso. (I use it with an Edelrid "Bulletproof" HMS karabiner which has a stainless steel inlay for the surface exposed to the rope, so this doesn't wear either.)
I don't usually climb with natural gear on a single rope, but if I did, then I guess I would use the bigger ATC Guide or (because I have already got it) a Reverso.