Pretty much any Zigbee device which plugs in can be used as a repeater. The first generation smart things motion detector which plugged into USB power is popular for this but you can use anything the plug then.
There are also “appliance modules” which are plug-in receptacles that act as repeaters and although these cost a little more, they may be usable as the device as well, so you get double use from them. These are mostly what I use.
In early 2015, we thought that zigbee bulbs if connected directly to the smartthings hub would be able to use as ZHA repeaters, but many community members have reported that they just seem to be unreliable and this was later verified by smartthings engineering staff. They were part of the time, but some messages seem to get lost. So at this time, I couldn’t recommend a zigbee bulb attached to the hub as a repeater.
(Note that the Phillips hue lightbulbs, even though they are Zigbee, do not act as repeaters for anything except other lightbulbs on the same Hue bridge if they are connected to the Hue bridge.)