LimeWire and other file-sharing software are legal. Using it to upload or download copyright protected media -- music, videos, images, software -- without the copyright holder's consent is illegal because it is theft.
LimeWire is completely legal. However, the purpose many people use it for is not. Downloading copyrighted music, videos, or programs without the permission of the author is illegal. This is called "copyright infringement" or "piracy." Calling LimeWire illegal would be like calling cars illegal because one could use it to run over somebody. It is just a tool.
Crystal