New England is a region legendary for its rocky soil. The farmers who originally settled here toiled for countless hours to clear the land of all those stones. Later, they (or their descendants) built beautiful stone walls from them.
Perhaps it is because I am a native New Englander that I have such an appreciation for the artful use of stone in a landscape design. When driving through the countryside, I always do a double-take if any stone wall strikes me as being aesthetically pleasing above and beyond the average. But stone walls can be functional, as well as pleasing to the eye. When do-it-yourselfers have slight slopes to tame, they can save some money and build small stone retaining walls on their own to meet the challenge, rather than hiring professionals.
Stone retaining walls can also complement rock gardens beautifully. Use stone retaining walls as "planters" later, by tucking small perennials (such as some of the smaller sedums) into the cracks between the stones.
Read tutorial: How to Build Small Retaining Walls