It doesn't often come up, but I generally use a variation of English common law.
There are eight common law felonies.
1. Murder
2. Rape
3. Robbery (requires the use of force or threat thereof)
4. Burglary (defined as entering the dwelling of another to commit a felony therein)
5. Arson
6. Grand Larceny (theft of goods worth more than 10 gold pieces)
7. Mayhem (wounding another so severely that the other cannot serve in the king's army)
8. Kidnapping
I add a ninth: Ensorcellment -- defined to mean using magic to compel or alter the behavior of another. Casting Charm Person against a shopkeeper is ensorcellment.
The penalty for all felonies is traditionally death. Hard labor, branding, or exile may be imposed as an alternative.
There are two common law misdemeanors.
1. Battery (the use of force against another not resulting in murder or mayhem)
2. Petite Larceny (theft of goods worth 10 gp or less)
The penalty for a misdemeanor varies, but is usually a fine.
Laws apply to interactions between civilized members of society. In other words, killing a human in a bar brawl might be murder. Killing goblins out in the woods is not -- goblins are inhuman monsters and it is fit that they die. Charming a merchant is ensorcellment and might get you burned at the stake. Charming a deranged, devil-worshipping cultist is just and prudent.
Law enforcement is inconsistent at best. Most towns and cities have a watch or guard, but these entities don't investigate crimes. They exist mostly to keep the peace by breaking up bar fights and keeping an eye out for trouble.
Courts in civilized areas are usually administered by the local lord. That is, the nearest Baron is the judge. There is no generally recognized right to a trial by jury. There is no general right to appeal, although powerful or influential defendants may be able to appeal to the King or the presiding Baron's Duke. Penalties and rules of procedure can vary significantly based on the temperment of the presiding Lord.
