The neighbor's dog barks at your front door all night long. Or maybe the hallway outside your apartment is busy all night long. Perhaps you like to practice your drums in the middle of the night. In any case, you need to provide some sort of sound proofing for your door. Here are some steps to follow.
Change the door. Most interior doors are hollow core and don't stop sound. If you rent and the landlord doesn't want to change the door, ask if you can change the door while you live there and then re-hang the original door when you move out.
Replace the door with a medium-density fiberboard door. They are not expensive and they do stop a lot of sound. There are also special sound-proof doors on the market.
Create a better door seal. Attach a metal door sweep at the bottom of the door. It not only keeps the cold out, but it blocks noise as well. Set the sweep to just barely brush the floor as it closes. Also put foam seals around the doorjambs.
Hang a specially made sound-proof doormat over the existing door. This works best if the door is flat. Depending on the doormat you choose, you can attach it with adhesive or peel-and-stick backing.
Look behind the molding in the frame of the door. If there is no insulation or filler there, then it's not sound proof either. There are sound-proofing caulks available to use in spaces such as this.
If possible, hang two doors from the same doorjamb. This pocket of air between them creates a dead space that sound proofs your door.