How CellSpotting works is really simple :

“A mobile phone is a radio device, when turned on, it is in contact with a “cell”. A cell is the smallest geographic area covered by a base station in the mobile network. The size of a cell can be from 100 meters in urban areas up to a few kilometers in rural areas. Each cell can be identified by a number, the “cellid”. Cells are also grouped into areas and the cells and areas are operated by a Mobile Operator. The cellid,area and operator is known by the mobile phone, and can be read using special applications from many mobile phones. The idea with CellSpotting.com is to read the Cellid information from the phone, use it as a “key” to look up the geographic location of the phone. If the cell is not known to the CellSpotting server then the user can assist, and help to provide with geographic information, it can can be a street address or some other information regarding the location of the cell. Is the cellid really unique, what about areas and operators? True, the Cellid is a number (16bit) and is only unique within an Area, but together with the areaid and the network code, they form a 3 part key and can uniquely identify the cell.”

download the cellspotting client for your phone and become a DISCOVERER right now!
oh, actually the use of the service is free of charge but the only way to use it is to have a modern phone such as a Nokia Series60 phone (7120, 3650, N-Gage)