My first thought was an Australian Shepherd but they do need alot of stimulation and excercise. Great dogs, though
How much excercise can you provide? How long are you home? Do you have a fenced in yard?
Those are all questions you should ask yourself.
Some other breeds I can think of right now would be the American Pitbull Terrier, Shetland Sheepdog, Boxer, Airedale Terrier, American Eskimo Dog, Chow Chow, Dalmation, Greyhound, Miniature and Standard Schnauzer, Samoyed, Schipperke, Brittany, and Whippet.
Do you want an active, energetic dog? Or a calmer, quieter one? Do you prefer certain coat legnths, colors, or patterns?
I'd start by going to akc.org and looking at all the breeds
Oh, and here is one of those selector things that say what breeds would fit in best with your lifestyle: http://www.pedigree.com.au/select/
Of course, there are always shelter mutts, too