Vigilante justice might be exhilarating, but it's got its drawbacks -- the police resent your efforts, and the people you save are constantly yanking on your ponytail. Clean up the streets from the safety of your own keyboard, with See Click Fix.
Founded in New Haven, but just-launched and "particularly useful" in SF, SCF's a mashup that objectively catalogs "non-emergency neighborhood issues" (e.g., graffiti, potholes, "a very unstable person...lying on the ground, ranting and apparently not doing well"...), and gives anyone heroic enough to have internet the chance to report on, monitor, or fix them. To report an issue, pinpoint its location and write a summary/suggest a fix; alternatively you can "watch" all of any given area's tickets by cordoning it off with a polygon (note: "watching" Tenderloin-shaped polygons is a full time job). When anyone comments on your ticket/your area's tix (e.g., "more graffiti's been added", "don't touch my graffiti"), or sees an issue's been fixed and closes your ticket(s), you'll receive email notification, which you'll want to print out as proof that, "I took a bite out of non-emergency neighborhood issues".
SCF also hassles private industry into monitoring the footprint it leaves on the cityscape, as when AT&T used it to relocate and wipe down huge eyesore cable boxes -- proving that not even multinational telecommunications giants are Above the Law.