Good scissors
Sharp knife
Burnisher
Stitch marking wheel
Awl
Replacement awl points
Heavy thread or rawhide
Needles, and not your basic hand sewing ones either, leather-working needles
Stamps and other tools if you want to put designs on the leather (whole 'nuther thread on which stamps!)
Stain for the leather
Hammer, preferably with a brass or rawhide head
Block of marble or hard stone to hammer on
Stitching pony is not required but really handy
What am I forgetting?
Oh, yeah, leather (another whole thread on this topic, as leather comes in different weights, finishes, etc. for different uses.)