Question
I have a a flat lawn area roughly 10-12 feet in depth followed by a steep hillside with a large tree and woody bushes and other vegetation.  It was left undisturbed during the development by the builder. The slope is greater than 25% and 30 ft and benched about 2/3rds up.  
Unforunately, my neighbor(a townhouse) completely stripped the area behind her portion of the property of all trees and all vegetation and installed a chainlink fence along the hillside.  
  
She replanted with small ornemental shrubs but there is still a considerable amount of exposed ground.  The hillside is mostly clay and shale.  She has been stripping 1 1/2 to 2 feet along the fence line on my property for several years.  I have had a considerable amount of erosion, rocks shale and dirt accumulating at the foot of the hillside where she stripped along with significant amounts of water pooling on the lawn (2 to 3 inches).  I took legal action to make her stop removing the vegetation on my side of the property.  What could I plant to prevent more erosion and control the water runoff that would require little maintenance?  The are gets good sun and faces west.  Thank you

Answer
Good for you in your ability to stop someone from being a nuisance.  I wish more folks would have the backbone to tell the neighbors to stop dumping their water & dirt onto other people's property.

Having said that, just as you found that actions by your neighbor affected your land, your potential solutions may create problems for your neighbors DOWNHILL of you.

With steep slopes like yours, mechanical devices are sometimes required, as your terrain probably exceeds the ability of a plant to put down roots and actually hold the hole thing in place.  There are lots of options, and it is more than just what is going on with YOUR slope.  

I would advise you to consult a local civil engineer (also called a geo-technical, environmental, or construction engineer), or landscape architect who would know better your area.  As you have indicated, there are local soil issues.  Plus, there may also be concerns over where the water is coming from, how it could be channeled or intercepted, and finally what might grow in your area that could work, if you insist on a "plant" solution.  

This site is for Virginia where it rains 60"/year:  http://www.ext.vt.edu/pubs/envirohort/426-722/426-722.html

Where I live, it rains 7"/year...where you live it could easily be 5-times that amount, requiring a different solution.  Maybe you've got an underground spring feeding the problem.  I just don't know, whereas a local professional would have the resources to help you better.

Professionals that may be able to assist you can be searched for at:

http://online.asla.org/scriptcontent/index_find_firm.cfm

 or

http://www.asce.org/inside/sec_brnch.cfm

Or the local phone/web book, if your community is large enough.

Hope it helps
~M