Tube or ATC. Tubes excel at giving a dynamic catch in case of a fall. Normally, they can be used with both single or twin/half ropes. They also work as descenders, although a prusik knot is always necessary. Very simple, versatile and cheap.
Assisted braking. Assisted braking devices are very popular in sport climbing. They are designed to lock in case of a fall and assist the lowering off the leader.
Plates. These are very simple devices that have stood the test of times and have been designed specifically for belaying a second and rappelling. Paired with a classic tube they're arguably the lightest and most versatile belay/rappel kit for multi-pitching.