Petzl Reverso 4

Backcountry.com says:
Not only does the Petzl Reverso 4 Belay and Rappel Device give you serious stopping power when you're holding massive whippers, but it also functions as an auto-locking device when you're belaying the following climber directly off the anchor. This allows you to re-rack, tie your shoes, sip some water, and take a few photos without risking your climbing partner's life. Belaying two followers at once' No problem. Rappelling 15 pitches' The deep grooves give you plenty of control. Petzl also added a secondary clip-in point to help you release a locked rope if you need to lower a follower who has fallen.