A land scanner is an object that houses a script which scans for public or very low-priced land, often used by land barons. When first found to exist, some residents found them horrible due to their "underhanded" practices and others questioned their impact to the sim's server.
Many topics ensued on what they did exactly and how it affected the sim they were in. Some questions raised were answered by some (but not all) owners of such objects. Soon it was found that many different types of land scanners existed: those that scanned a sim from the owner's parcel, those that roamed the grid high in the sky, and those that spawned copies of themselves.
Most recently (date needed) it was found that there were self-replicating land scanners, which would create many copies of objects which would then fly about the world looking for certain land. These scanners somewhat affected the sim server negatively and also had no permission to be on the resident's land in which they appeared. Often undetectable by the naked eye, land scanners can be seen using scripts or the "Highlight Alpha" view menu option.
Many land scanners still exist, but non-script-savy residents, who don't want to make such things, have been given a slight upper hand when Linden Lab made a webpage which displayed public land and where it's available. However, residents have pointed out that land barons also most likely use this to their greater advantage.