Steve McQueen starred in a 1968 cop film called "Bullitt" where he drove a fastback, hunter green colored Mustang. The movie gained high popularity due to its car chase which featured this high-powered Mustang. In 2001, Ford attempted to relive its glory from this movie by making a few limited edition Bullitt mustangs. This car, in 2001, had a few upgrades from the GT's such as a new intake design, lower to the ground suspension of about 3/4 in., and advanced upper drive pulleys.
Basically, this car was a beefed up version of the GT, and only offered in a hunter green color, black, or true blue. But very few true blue mustangs exist. The majority bullitts came in this dark, highland green color.
Ford continued this tradition of making bullitts again in the 08-09 range. They, too, had upgrades from the GT's. These cars are rare, but unless you get one in mint condition, I really don't think it'll be a car that will appreciate in value like the 2000 Cobra-R will.
Hope that answers your question