A Bit About Halaal (Muslim dietary restrictions)
Because of the purity requirements of these foods, they tend to be a) fresher, and b) more healthy. According to what I could find on the web, the laws come from the Holy Koran, the Hadith, and the teachings of Hanafi, Shafii, Maliki and Hanbali. If you eat any (and they really mean any) Haraam foods, it affects the acceptance of worship. In a nutshell, the forbidden foods are as follows (source: www.jamiat.org.za):

• Pork and pork products are totally forbidden.
  
• Dairy products like cheese and margarine made from plant, microbes, synthetic rennet are okay
  
• The utensils should be separate for Muslims and non-Muslims
• Any item marked or stamped with any form of Halaal certification must be verified by Appointed the Muslim theologians well versed in Islamic requirements regarding Halaal
• All carnivorous animals and birds, e.g. lions, tigers, vultures, eagles, etc.
• Any Halaal animal that has died due to natural causes, killed by another animal, by a fall, or slaughtered by a non-Muslim
• Flesh that has been sacrificed for some god or goddess
• All forms of najasat (impurities)
• Drugs/intoxicants
• Animals that aren't traditionally used for food, donkeys, monkeys, elephants, etc.
• Food containing Haraam items to be avoided at all times e.g. fish prepared with wine, cakes and ice-cream containing liquor
• Contamination of a Halaal item with non-Halaal will render it non-Halaal/Haraam and unacceptable.

Foods that are allowed as long as it has been slaughtered by a Muslim according to Islamic laws:

• Fish that has been caught, i.e., not one that died a natural death
• All types of poultry, partridges, pigeons, etc.
• Goats, sheep, cattle, camel and all types of buck, buffalo.
• Rabbits

The interesting thing is that as new foods are developed, the laws evolve, e.g., the rules about vanillin, a relatively modern synthetic.