Molded composite foam rollers are perfect for positioning, balance, postural and muscle re-education, spinal stabilization, body awareness and coordination, and ranging and strengthening activities.

ProSource High Density Extra Firm Foam Roller for Muscle Therapy and Balance Exercises

Although these come in various shapes and sizes, I personally have the extra firm foam roller.  Using just body weight, user can lengthen muscles, and decrease spasms.