RCTgo

There are many good rock sets out there including the one nathan8848 mentioned. Here are the links to the sets I like to use:

Weber's Gazebo and Rocks, Weber's Boulder and Brush, Weber's Stone Creations Volume 1-3, Deep Caverns, Canyon Craze, Large and Medium Rocks  http://velvetskyemusic.com/RCT3_files/cso.html

Krypts Galaxy has a few different rock sets as well http://krypt.weebly.com/custom-scenery.html