ESP-Now
Updates:
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If you're looking for something like ESP-Now but with instead of ESP to ESP with ESP to something else like a Raspberry Pi then checkout this alternative adhoc networking approach.
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As of 19 Aug 2018 the latest ESP8266/Arduino code has fixed Issue 3408 and also added a deepSleepInstant function. This means an ESP-Now sensor can do the wakeup-send-sleep cycle in around 40 milliseconds, about an order of magnitude faster than previously and it should now get really good battery life. I've added a minimal example demonstrating this in espnow-sensor-minimal ***
ESP-Now is an interesting ESP8266 protocol that could be used for battery powered sensors. Its built on top of 802.11 vendor-specific action frames which enables sending data without having to first establish a WiFi AP to Station connection which is time consuming and so adds significantly to draining the batteries on each sensor wakeup.
I've given some examples of it before, here, however ESP-Now doesn't work with Wifi so a gateway receiving ESP-Now transmissions can't also connect to a Wifi network, so thats made it pretty impractical for most uses.
I've been playing about with ESP-Now again recently and come up with two workarounds for the Wifi co-existence problem. One is to restart the ESP8266 after receiving each ESP-Now transmission, the other is to use two ESP8266's - one as the ESP-Now receiver and the other as the Wifi gateway.
The first approach of restarting the ESP8266 might seem a bit clunky but it actually works ok. The restart takes a few seconds (most of the time spent reconnecting WiFi), so if there aren't too many remote ESP-Now sensors or the sensor transmissions aren't too frequent then the Gateway being regularly unavailable for a few seconds doesn't actually matter.
The second approach with using two ESP8266s is more complicated to set up but also more robust and allows higher throughput. I've used a simple serial link with the ESP-Now receiver simply writing the received data to the Serial port which another ESP8266 can read using SoftwareSerial. You could also try writing a more fancy I2C or SPI driver too.
These are some ESP-Now devices using this approach. A battery powered temperature/humidity/pressure sensor built into a LED light case and an ESP-Now to Wifi Gateway using the DIY USB powered ESP (described here).
Sample code for these are in this repository.
You can see the data from that ESP-Now based BME280 sensor thats in my back garden in London here.