I grew up in an area so flat that people created berms in their yards just to provide a little contrast.  However, several years ago my sister and I were faced with the problem of how to deal with the fast-eroding slope behind our summer cottage.  We didn't have the money to terrace the slope, but needed a relatively cheap way to stabilize the soil. 

Part of our slope was sunny and part was shady.  The soil was like concrete, because most of it was concrete--or at least "clean" fill dumped into place after we had the rear of our cottage renovated.  Any plant that required a hole more than a few inches deep was out of the question, as were plants that required maintenance. 

We decided on a combination of daylilies for the sunny areas and hostas for the shade.  Both plants have tough roots that hold the soil and a reputation for withstanding bad conditions.  We bought the most inexpensive daylilies that we could find and saved money by buying in quantity.  The hostas were transplanted from other areas on the property.  Installing them was torture--more like mountain climbing than gardening--but we persisted until we had daylilies and hostas spaced at regular intervals up and down the slope.  We mulched them as best we could with the "seaweed" that washed up on our lakefront beach.  We watered regularly throughout the first summer.  We figured if the plants failed, at least we bought ourselves an extra year to save up for the terracing project.

Much to our suprise, the daylilies and hostas took to the miserable conditions in a big way.  Not a single plant died and all increased in size.  Now, after several seasons, the slope is completely covered.  The rainstorms come and go, but the soil stays in place.  People in boats slow down to see the spectacle when the daylilies are in bloom.  For some reason, the deer, which are plentiful in the area, do not eat either plant.  (It's probably because the slope is so steep that even they dislike trying to eat on it.)   

So if you have a lot of slope and only a little money, take heart.  Find a tough-rooted, fast growing plant that isn't poison ivy or kudzu and you will have found the answer.