Smoke Sensor - MQ-2: www.sparkfun.com/products/17049
Dual Gas CO and CH4 Detection Sensor - MQ-9B: www.sparkfun.com/products/17050
Low Concentration Ozone Gas Sensor - MQ-131: www.sparkfun.com/products/17051
Hydrogen Sulfide Gas Sensor - MQ-136: www.sparkfun.com/products/17052
Ammonia Gas Sensor - MQ-137: www.sparkfun.com/products/17053

The MQ-2 Smoke Sensor can detect flammable gas in any range from 300 -10000ppm, and the heater uses 5V.
The MQ-9B sensor’s conductivity gets higher as the carbon monoxide and methane gas concentrations rise. The heater uses an alternating voltage of 5V and 1.5V, depending on the gases. If you’re only testing Carbon Monoxide, the heater can be set to 1.5V. It has a sensing range of 10-500ppm for methane.
The Low Concentration Ozone Gas Sensor is highly sensitive to ozone, also known as trioxygen, and oxides like Chlorine & Nitrogen Dioxide. Its sensing range is 10-1000ppm, and the heater uses 6V. The load resistor for this sensor is a lot higher than the other sensors, 100/200k, and it’s incredibly sensitive. It measures in parts per billion rather than parts per million.
MQ136 & MQ-137 both have heaters that use 5V. MQ-136 senses hydrogen sulfide, and MQ-137 senses Ammonia.