In the US, there are a lot of choices. Lowes Iris makes one which works as both a zigbee repeater and a Z wave repeater (that’s very unusual), so it’s popular for that reason. But check to make sure it is the 3210 model, they make a bunch of other plugs as well that don’t have the same features. It will end up showing up as two devices in your smartthings account. It will be a controllable on/off switch as a zigbee device, and it will be a zwave repeater (but not controllable) as a separate device.
Whilst the plug itself can only be controlled over Zigbee, it also acts as a Z-wave routing device. This means it can extend your Z-wave network range, but cannot be controlled via this network.
Osram/Sylvania makes both a US model and an EU model of their pocket socket which tend to be quite inexpensive and works well:
And in late 2018, Ikea introduced a pocket socket in their Tradfri line which is a simple basic on/off switch with one socket that works well as a zigbee repeater with smartthings. The product description will say that it needs the tradfri gateway, but you can use the smartthings hub instead.
At the time of this writing, It also works well as a repeater for Xiaomi sensors, which many other brands do not.
Cost is only $9.99 in the US and £9.99 in the UK. It is also available in other European countries with the local plug format. The tradfri bulbs Are also good zigbee repeaters, and those are on the official “works with smartthings“ list.
note that the handheld buttons and remotes in this line do not yet integrate fully with Smattthings, but the bulbs and the pocket socket do.
But there are a lot of brands.