These are all new member freebies, and the sites are free to join:
ArtsCow 100 free 4x6 prints, and 50 free 5x7 prints each month until free period expires, plus lots of other photo goodies
Shutterfly 50 free 4x6
Snapfish, 20 free 4x6, just click 'sign up'
Clark Color, 20 4x6
If you know of any other deals, just leave a comment below and I will add it in:)