(US, UK) Swann One Key Fob Remote Control SWO-KEF1PA, Zigbee ZHA 1.2
This is a nice security system keyfob intended for their own Gateway but which will also work with the SmartThings classic app with the right DTH. You can also find it quite inexpensively at times, in the under $20 range. And most of the rest of the time it will still be under $25. The manufacturer’s list price is $39.
US Listing:
https://www.amazon.com/Swann-SWO-KEF1PA-One-Key-White/dp/B00VAIKO66
UK Listing:
Discussion thread with DTH:
SwannOne Key Fob Support (updated 2019 DTH in post #51)
Some people bought this device back in 2014 or 2015 and it didn’t work with smartthings at that time so they had to return it. However, there have been a number of changes to smartthings’ zigbee implementation since then, and as of 2019, it does now work. 
Note, however, that this cannot be used to arm/disarm the official smart home monitor feature in the “smartthings (Samsung connect)” Version of the SmartThings mobile app.
If you want it to change smart home monitor armed states (security.mode), that will only work with the smart home manager feature in the classic app, and in order to make that happen you have to use a routine.
So if you are looking for something to activate the “security.mode“ options, be aware that this may go away in the future when the classic app is discontinued. (There has been an official statement that it will be discontinued, but not when.)
Otherwise, it should work in the Classic app like any button controller with a custom DTH. You can use it in the official smartlights feature, webcore, or other Smartapps that recognize button devices.
Please don’t ask any further questions about the differences between the two apps or the future sunsetting of the classic app in this thread, which is a device thread. If you do have questions about all of that, take them to the following thread:
"SmartThings Classic" (V2 app) vs "SmartThings (Samsung Connect)" (V3 app)
If you have any further questions about the swann remote, take those to this thread: