Pepper "hotness" is measured in Scoville Heat Units (SHU). For a comparison, pepper spray (the stuff you spray on an attacker, not in him) is measured at 2,000,000 - 5,300,000 SHU. A plain old bell pepper is measured at 0 SHU. Tabasco Sauce is 2500-5000 SHU. Hopefully that gives you a sense of scale.
Here are the hottest peppers in the world:
Naga Jolokia (naga morich, bhut jolokia) 1,040,000 SHU
Dorset Naga (Capsicum chinense) 970,000 SHU
Red Savina (Capsicum chinense Jacquin) 577,000 SHU