Scoville Scale

In 1912 Wilber Scoville developed his now famous method tho chart the comparative heat of different chillis. (J. Am. Phar. Assoc. 1912; 1:453-4) The greater the number of Scoville Units, the hotter the chilli. For example:

Bell Pepper................0 Scoville Units (SU)
Peperocini, Cherry Pepper..............100-500
New Mexico, Aji Panca.................500-1,000
Ancho, Passi, Espanola...............1,000-1,500
Sandi, Rocotillo, Cascabel, Poblana..1,500-2,500
Jalapeno, Mirasol....................2,500-5,000
Chilcostle, Louisiana Hot...........5,000-10,000
de Arbol, Serrano, Japones.........10,000-30,000
Piquin, Aji, Cayenne, Tabasco......30,000-50,000
Chiltepin, Tepin...................50,000-80,000
Habanero, Scotch Bonnet...........80,000-300,000
Pure Capsaicin........................16,000,000

This can only be a rough guide, since the heat of chillis can vary from pepper to pepper.

Source: Schott's Original Miscellany by Ben Schott