Somewhat less popular than the traditional standing calf raise, the donkey calf raise similarly targets the gastrocnemius muscle of the calf. Interestingly, the use of iEMG (which is used to record muscle activity) shows the donkey calf is slightly more stimulating to the gastrocnemius than the traditional standing calf raise.
This exercise profile discusses the donkey calf raise performed using the machine, but the exercise can also be performed without one. Although less common these days, the athlete can perform the exercise by using a raised platform, a sturdy upright for support and a training partner who is will to sit on their back. Obviously this choice is not suitable for all, and will probably draw some attention in many commercial gyms!