OpenCellID is providing a very unique and exciting open source project. It's aiming to create a complete database of CellID worldwide and their locations. This project is lead by 8Motions.
What exactly is a CellID? A CellID is the unique number of a GSM cell for a given operator. Your phone is always connected to a Cell, and by knowing this number, you know the Cell, and it can translate to know where the phone is physically located.