What can I use to switch a 500mA 12v inductive load from 3.3v Output
You want to choose a MOSFET with a low Gate threshold voltage as your logic level is limited to 3.3V, at 500mA there are alot of options including:


IRLML2803TRPBF
NDT3055L
NDT456P


Just to name a few, you probably want to add a flyback diode as I assume the valve will uses a coil that gets magnetised to engage the valve. 



You do not need a current limiting resistor on the Gate of the MOSFET as MOSFETS draw almost no current into the gate. A pull down resistor is useful so as to ensure the gate is at a known state when the output of the ESP8266 pin is either disconnected or is in a high impedance state. A 10k will suffice as it is a strong enough pull down but will not draw too much current when the gate is high. 

(The circuit shown above uses an N channel MOSFET and will be engaged when the gate signal is high, if the reverse is required, a P channel should be used.)