Rabble-rouser: someone who rouses the rabble. Sometimes written rabble rouser or rabblerouser, it's one of those uncommon compound words that nobody's sure how to spell. Offically it is written with a hyphen. (Yes, I looked it up.)
The rabble are a "tumultuous mob" or "disenfranchised lower class". The key is that they are disorganized until someone (the rabble-rouser) instills in them a common goal or sentiment that leads to a disturbance. The result can be protest, riot or some other thing the upper class doesn't like very much.
The term rabble-rouser is not used very much today particularily because the idea of social class has changed: there is no longer a bona-fide aristocracy. On the other hand, leaders of environmental or anti-globalization groups could be considered modern day rabble-rousers if we view corporations as the ruling class. These group leaders try to instill in the modern "lower class" (teenagers and gen-X'ers without stock options) a sentiment of anger towards the corporate behemoths, leading to unintentionally violent protests. Whether or not this is a good thing is debatable.