Dynastree (formerly ItsOurTree.com) has a new tool that shows you where others with your last name live. You simply type in your name and it shows you on a state-by-state basis where there are others who share it. Of course if your last name is "Smith" or "Jones" this won't help much, but I found it to be spot on with mine.
Besides the U.S., which has approximately 1.7 million last names in its database, you can also search all of Canada. Included with both is a small analysis right below the map that ranks your last name's prominence in each state (or province/territory), as well as on a national level.
Where the tool won't help you is tracking some of these folks down, or helping you figure out if you're actually related to them. That, to me, seems like the next logical step.