To summarise, there are two requirements in this thread :
1-) From ST ==> ESP8266 : ST to control via wifi ESP8266, which relays some “Thing”. This would be pretty useful in many applications (certainly excellent possiblities but maybe not power efficient since it means that ESP8266 need to listen at all times, draws power. But really worth it for its low cost if it can be powered with a source) ;
2-) From ESP8266 ==> ST : ESP8266 to be able to trigger on ST via WIFI .
This second possibility opens a whole whole world to low cost possibilities ( since most z-wave devices start at minimum 20 USD /thing). The famous Amazon dash button ( which is 4 USD can be easily adapted to turn into a switch button for your SmartThings. Dimmers I am sure can, and the start would be 4 USD / piece instead of minimum 20 USD item presently with Z-wave. Energy will not be a problem in this case, since ESP8266 will be ins leep mode most of time untill it wakes up further to a push button or so.
The question that remains unclear to me is whether there is a ST device type , which can listen on local TCP port on specific URL to trigger some event. That’s all we would need to make option 2 work.
Any lights on this ?