The City Repair Project, based in Portland, Oregon, provides inspiring examples of people organizing community and physical improvements in their neighborhoods. You can view many of these projects on their website.
City Repair's most well-known work involves what they call "Intersection Repairs," a process of organizing on a block and creating such installations as a huge painting on the pavement of their intersection, a corner book lending hut, a neighborhood composting station in the median of the street, and so on. However, their years of experience have much to offer us even if we are only planning to reach out to some neighbors and maybe throw a block party.
You can read sections of their excellent City Repair Guidebook by clicking on the cover image to the left, and you can also order the Guidebook on the City Repair website.