Pachelbel's Canon is (no surprise) number 1 on the list (as well as #7 and #46 due to some common misspellings). I image that it will always be at the top of this use-generated list because its perennial use in weddings. I have been playing weddings for the past 15 years, and it is very rare that a bride doesn't want to have the Pachelbel Canon played. (It happens to be a very useful wedding processional because it is possible to tailor it on the spot to be any length and to fit any "mood." In spite of what some people might say or how "tired" some people might be of it, I think it is a remarkable piece.)
I was not surprised to see the "Lacrimosa" from Mozart's Requiem in the number 2 spot because of its exposure on the basketball shoe commercial, and I was not surprised to see a bunch of pieces that are used in many of the standard music appreciation textbooks. I was not surprised to see music used in a few Star Wars films, but I was surprised to see Shostakovich's The Gadfly and Five Days-- Five Nights as well as some Debussy Preludes.
The labels on this list are sometimes incorrect. Young people often confuse the composer of a piece with the performer on a recording, hence the use of that all-purpose (and misused) word "artist." Some of the labels on this self-generated list confuse the "artist" with the composer. Maybe the people at Last FM or Technorati will fix that and make the resource even more useful for people new to classical music.