In the halves, the common ones without mint marks are from Philadelphia's mint. They didnt start using a mint mark ("P") until 1980. "D" is Denver, "S" is San Fransisco where they do only Proofs, and "W" is from the West Point, NY mint but they only do liek the bigger collectible ones liek the $10 Eagles and such. If you get lucky enough to find a Barber or Liberty halve with "O" that was from the New Orleans mint and "CC" was from Carson City.
So there you have it... mint marks in a nut shell.