Cruise down the tree-lined streets of the Old Oaks neighborhood in Columbus, Ohio, on a summer evening and know this: Someone is watching you. It might be Richard Vickers, who records your license plate number in a notebook as he retrieves gun shell casings from the sidewalk while out on his nightly walk. Or it might be Doug Motz, who alerts via text message: “Watch out for the green van lurking in the alley.” Like the members of this well-oiled block watch group in central Ohio, neighbors across the country are using Twitter, blogs, e-mail and street patrols to help thwart crime.