Boobs, breasts, tits, titties. Preferably naked ones.
From Sexoteric:
Being tied to a wall does wonders for this lady’s breast posture:
From Breast Spanking.
From FEMJOY:
From ErosBlog, of course:
Update: Now, more Sophia Loren titties!
Here’s Japanese idol Kaori Mochida enjoying a spot of breast bondage: