Walmarts, some K-marts, and Cracker Barrels work for us as well. Like JDUB, we've seen folks who wear out their welcome. Since some WMs have changed their overnighting policy as a result of these weirdos, we make it a point to call ahead or check with the management before opening the awning and lighting the Barby (just kidding) .
Considering all the aviators in the crowd am surprised no one has mentioned overnighting at small airports. They are usually very quiet, have good parking lots and have been very accommodating to our requests for an overnight stay.