Many printing services offer free photo prints so customers can test the photo quality. A list with free offers can be found here.
If you know an offer which is not listed here, please tell us so we can add it.
Foto.com:
10 prints in size 4x6 free on every order
Snapfish:
20 free prints for new customers
Kodak Gallery:
20 free photo prints when signing up
Shutterfly:
15 free photo prints for new customers
