n0tice is a simple location based public notice board system. It's completely open and unfiltered, and allows users to post any sort of notice they want, tagged with their location, and lets them read notices near them, sorted by distance. Quite a simple little idea, and if it gets a user base, it could end up being quite useful. Although first designed for iPhone, the n0tice API was quickly released to allow integration into other programs, including desktop apps.
Quick: Name that professional network with more than 50 million members. Did you guess... Viadeo? LinkedIn is larger, with over 175 million members. But Viadeo has upped it is game with its acquisition of Pealk, which bills itself as the #1 head hunting App on the web. According to the press release, that search ability is what Viadeo is after.