The most helpful local site to find leftwing protests is IndyBay which has a calendar. There's other stuff too, often blurring the line between political and just fun activities (like free showings of political movies). Other cities have similar websites, I'm sure, if not quite so extensive. A lot of items on IndyBay's calendar seem to really get filled in just a couple of days in advance, although you can learn the dates of many activities months in advance because their planning meetings show up on the calendar as well. Once you find a cause or an organization you find particularly fascinating you can subscribe to their mailing list. You probably don't have to subscribe to all of them - as an example if you're mostly interested in going to the major anti-war protests if you get on the lists of any of the larger organizations they're going to have a group there. It's fascinating to see their "news" coverage after an event, which often bears little or no resemblance to anything that actually happened.
Also, just keep your eyes open at local cofee shops or on campus and you'll see this kind of thing:
There are lots of photographers who do protest photography as an occasional thing, but these people have had a real focus on it.
Zombietime - this is a really massive website of a photographer who is also in the San Francisco Bay Area who shoots protests. Luckily (?) there are so many events around here to cover we overlap less than you would think.
Urban Infidel - in the form of a blog, but the same sort of deal - she documents protests, but in her case in New York.
Ringo's Pictures - he has some wonderful current work and is adding in some older stuff and it's well worth a look. So far it's all in Southern California.
You might also want to keep an eye on Michelle Malkin who often links to protest reports that interest her.
You can contact me at info (at) ProtestShooter.com