The lower half of Apollo's 11 LM (the "descent stage") was left on the surface of the moon at the landing site, and the upper half (aka "ascent stage", which rendezvoused with the Command Module) was jettisoned and left to crash back on the surface of the moon. The exact current location of the ascent stage, however, is officially "unknown".

Source: http://nssdc.gsfc.nasa.gov/planetary/lunar/apolloloc.html