I don't know about gym exercises, but there's one I do at home that I like.
Stand on your toes on the edge of a stair and lift yourself up and down with your toes. This flexes your calves. I have well defined calves and this is the only calve specific exercise I do - although I do ride my bike a lot.
I like this exercise because you get to stretch the calve muscles out, since when you go down your heels are going below your toes.
If you try this exercise I suggest doing it in a stairwall where you can balance yourself on the walls until your balance improves enough that you can do it no-hands.
-Tamerlain