All the ones mentioned previously.
Beethoven's Sonata Pathétique.
Vivaldi's Gloria (if you like choral music).
Mozart's Turkish Rondo (aka Turkish March or Rondo alla Turca).
Bach's Air on the G String, and Prelude in C, bwv 846, from the Well-Tempered Clavier.
Also, Leo Kottke's guitar rendition of Bach's "Jesu, Joy of Man's Desiring" is
absolutely gorgeous.
You can also find classical guitar versions of Greensleeves, Moonlight Sonata and others.
Not really a classical song, but a nice instrumental, is "Ashokan Farewell" from Ken Burns' Civil War soundtrack.
BTW, nice sig for a guy asking about classical music.
