Download ProSurfaceSnap Free Unity. 

Welcome to ProSurfaceSnap!

ProSurfaceSnap is an editor extension that is incredibly useful for populating your 3D world with props. Ideal for terrain and mesh surfaces, with a very intuitive interface (mouse move to snap/rotate/scale/offset) this handy plugin will make level design and world decoration fast, intuitive, and fun!

Presentation Video
Forum

Video tutorials:
Quick Intro
Surface Settings
Per Layer Settings
Hotkey Customization