Retaining walls are usually ugly stacks of cement blocks, but they don't have to be an eyesore - for example, take a look at the Living Wall at the Natural History Museum of Los Angeles.
My family and I went there one weekend and we were pleasantly surprised to see the North Campus' asphalt parking lot has been replaced with a three-and-a-half acre of green garden, right smack in the middle of the urban sprawl of Los Angeles.
Next to the meandering walkway where we took a stroll was this marvelous living wall made from large shards of colorful Pritchard flagstone. The stones are arranged at haphazard angles, which allows for succulents and plants to be planted in the cracks. Soon, wildlife will start making the Living Wall their home.
Plenty of pictures over at Homes & Hues: The Living Wall at the Natural History Museum of Los Angeles