Not sure how well-known this plugin is, but I thought I'd spread the word since I use it often.
Here's the link.
Itoo, the makers of the Forest Pack plugin, also have another useful (and FREE) plugin called "Glue". What glue does is simple: "This plugin aligns the pivot point of an objects selection over the surface of another object. For example, it's useful in placing trees over a mountain. Also conforms a spline to follow the surface."
It's great for creating a road that follows topography.