A mobile app that finds my closest neighborhood block watch group, lets me join, lets me see which shifts are taken, and lets me sign up for watch shifts.
Make widgets (e.g. WordPress widgets) that let local blogs easily re-use and filter open data sets for display on their blogs. For example, show recent crime reports by zip code.
Get mobile alerts when a scam (like a door to door charity scam) is reported near me. Submit scam reports. Search recent and nearby scam reports.