Next to the previously discussed M3, there is also an ESP8285 M2 (in fact, there is also an M1). Like the M3 it is based on an ESP8285 that has 1Mb on board.
It has 11 I/O pins (including Rx and Tx) as well as an ADC.
An advantage though is that it is available on a development board that has a Vcc and Grnd connection per I/O pin (but read the caveat below), whereas e.g. the Wemos D1 mini only has 1 ground and 3V3 pin.
The board can be programmed directly from the Arduino IDE, but it does not have a circuit present that puts it automatically in flash mode. Therefore it is necessary to push the Flash button, hold it and then press the Reset button and only then release the Flash button.
There is a big caveat with this board though: The row of positive pins opposite the I/O pins, is connected to the INPUT voltage, so even if you feed this board via USB, you get 5Volt on the “V” pins. If you feed your board 3.3 Volt via the 3V3 connector……there is no voltage at all on the ‘V’ pins. Whether this is a design flaw or intentional I do not know.
Arduino, ESP8266, ESP32 & Raspberry Pi stuff 