One of the most successful ones I've seen is by Mary Brogger here in Chicago.  It's a memorial to the Haymarket Riot.  Here's a blurb from the DCASE web site. The base of the memorial is now partly covered by plaques and placards.  I can't remember if they commemorate other social justice movements or are of social justice organizations.  But what I like about it is that the memorial moves people to add onto it as a living shrine.

Lynn Basa