The Scoville scale
http://www.chilefarm.co.uk/scoville.html
The Scoville scale is a measure of the hotness of a chilli pepper. These particular fruits contain capsaicin, a chemical compound which stimulates heat-receptor nerve endings, and the number of Scoville units indicates the amount of capsaicin present.
List of Scoville ratings
Scoville ratings may vary considerably within a species—easily by a factor of 10 or more—depending on seed lineage, climate and even soil. This is especially true of Habanero peppers. A brief summary of some heat levels....
16,000,000 - Pure capsaicin
5,300,000 - Police grade pepper spray
2,000,000 - Common pepper spray
350,000 - 580,000 - Red Savina Habanero
100,000 - 350,000 - Habanero
100,000 - 325,000 - Scotch Bonnet
100,000 - 225,000 - Birds Eye pepper
100,000 - 200,000 - Jamaican Hot pepper
80,000 - Dave's Insanity Sauce
50,000 - 100,000 - Thai pepper
30,000 - 50,000 - Cayenne pepper
30,000 - 50,000 - Tabasco pepper
2,500 - 8,000 - Jalapeño pepper
2,500 - 5,000 - Tabasco sauce
0 - Sweet Bell pepper