Lots by Woody spring to mind, such as going Down The Road (I Ain't Gonna Be Treated This Way). Unlike a lot of more modern protest singers, such as Dick Gaughan and Christy, both of whom I love and have seen many times, Woody didn't generally deal in polemics, never pushing the protest down your throat. You can put the message across just as well, if not better, by simply telling it like things are or by putting across a story, Pretty Boy Floyd or Tom Joad...

This Land Is Your Land is a good protest song because it tells you like it is and how it should be, rather than singling out people to call bastards.