Below is a list of music that I would recommend for expanding your musical repertoire.  It is by no means comprehensive.  I have listed the English titles of such things as operas and cantatas.

Choral

Bach, J.S. 

• "Sleepers Awake"
• "Christ lay by death enshrouded"
• "St. Matthew Passion"

Beethoven

• Christ on the Mount of Olives

Brahms    

• Ein Deutsche Requiem

Faure      

• Requiem

Handel   

• Messiah
• Judas Maccabeus
  

Rutter   

• Requiem

• "A Gaelic Blessing"
• "For the Beauty of the Earth."

Wilberg 

• "Jesu, the Very Thought is Sweet"
• "Come, Thou Fount of Every Blessing"
• Requiem
• "Still, Still, Still"
• "The Lord is My Shepherd"

Solo piano

Bach 

• The Well-Tempered Clavier

Beethoven

• "Pathetique" Sonata
• "Appassionata" Sonata
• "Moonlight" Sonata

Chopin  

• Etude in g minor
• Raindrop Prelude
• Minute waltz
• Waltz in a minor

Grieg 

• Notturno in C Major

Liszt 

• "Un Sospiro"

Rachmaninoff 

• Prelude in g minor
• Prelude in c# minor

Scriabin 

• Sonatas for Piano

Solo violin

Bach    

• Partitas
• Ave Maria (Gounod melody over Bach Prelude #1 from The Well-tempered clavier)

Beethoven 

• Romance in F Major
• Romance in G Major

Lalo     

• Fantasie on Themes from Carmen

Massenet

• "Meditation" from Thais

Solo viola

Bach  

• Cello Suite No. 1
• Air on a G string
• Arioso

Bloch  

• Suite for Viola

Faure 

• "After a Dream"

Schumann 

• "Arpeggionata" Sonata

Telemann 

• Concerto in G Major

Solo cello

Bach 

• Cello Suites

Symphonies

Beethoven 

• Symphony No. 1
• Symphony No. 3 "Eroica"
• Symphony No. 4
• Symphony No. 5
• Symphony No. 6 "Pastoral"
• Symphony No. 7
• Symphony No. 9 "Choral"

Bizet         

• Symphony No. 1

Dvorak

• New World Symphony

Haydn       

• "Drumroll" Symphony
• "Surprise" Symphony
• "Departure" Symphony

Mozart

• Symphony No. 20
• Symphony No. 39
• Symphony No. 40

Schubert

• "Unfinished" Symphony

Shostakovich

• Symphony No. 5
• Symphony No. 9

Sibelius     

• Symphony No. 2

Tchaikovsky 

• Symphony No. 2
• Symphony No. 3
• Symphony No. 6 "Pathetique"

Memorable Concertos

Bach           

• Concerto for Two Violins in d minor
• Concerto for Violin in A Major

Beethoven   

• Concerto for Violin
• Concerto for Piano No. 1
• Concerto for Piano No. 4
• Concerto for Piano No. 5

Mozart

• Violin Concerto No. 3
• Violin Concerto No. 4
• Piano Concerto No. 20
• Piano Concerto No. 21
• Piano Concerto No. 23

Rachmaninoff

• Concerto for Piano No. 2
• Concerto for Piano No. 3

Ballets

Bernstein

• Fancy Free

Delibes

• Coppelia

Prokofiev

• Romeo and Juliet

Tchaikovsky

• The Nutcracker
• Sleeping Beauty
• Swan Lake
  

Other orchestral works

Brahms

• Academic Festival Overture

Ravel

• Bolero
  

Opera/Operetta

Beethoven

• Fidelio

Bernstein

• Candide

Bizet

• Carmen
• The Pearl Fishers

Gilbert and Sullivan

• HMS Pinafore
• Iolanthe
• The Mikado
• The Pirates of Penzance
• Princess Ida
• Ruddigore
• Trial by Jury
• The Yeoman of the Guard

Gounod

• Faust

Gluck

• Orfeo and Eurydice

Humperdinck

• Hansel and Gretel

Massenet

• Cendrillon
• Thais

Mozart

• The Abduction from the Seraglio
• Don Giovanni
• Idomineo
• The Magic Flute
• The Marriage of Figaro

Paisiello

• The Barber of Seville

Puccini

• The Bohemian
• Madame Butterfly
• Tosca
• Turandot

Purcell

• Dido and Aeneas

Smetana

• The Bartered Bride

Strauss

• The Bat

Tchaikovsky

• Eugene Onegin

Verdi

• Aida
• The Fallen Woman
• The Masked Ball
• Rigoletto

Wagner

• The Ring Cycle

Bach

• Minuet in G
• Well-tempered Clavier Prelude #1
• Zion Hort Die Wachter Singen

Beethoven

• Moonlight Sonata, 2nd movement

Copland

• Appalachian Spring
• Hoedown from Rodeo
• Fanfare for the Common Man

Gershwin

• Rhapsody in Blue

Haydn

• Surprise Symphony

Humperdinck

• Hansel and Gretel

Mozart

• The Magic Flute

Massenet

• Meditation from Thais

Pachelbel

• Canon in D

Prokofiev

• Peter and the Wolf

Silvestri

• Theme from Back to the Future
• Theme from Forrest Gump

Tchaikovsky

• Waltz from Sleeping Beauty
• Rose Adagio from Sleeping Beauty
• Dance of the Little Swans from Swan Lake
• Arab Dance from The Nutcracker
• Overture from The Nutrcracker
• Dance of the Snowflakes from The Nutcracker
• Chinese Dance from The Nutcracker
• Trepak (Russian Dance) from The Nutcracker
• Waltz of the Flowers from The Nutcracker

Williams

• Flying theme from E.T.
• Somewhere in My Memory from Home Alone
• Star Wars Theme
• Superman Theme
• Yoda's theme from The Empire Strikes Back

The Insomniac's Friend (Music to relax by)

Bach

• Cello Suite #1, 1st movement
• Concerto for Two Violins in d minor, 2nd movement
• "Zion Hears the Watchmen Singing"
• Prelude #1 from The Well-Tempered Klavier

Beethoven

• Pathetique Sonata, 2nd movement
• Symphony #1, 2nd movement
• Symphony #5, 2nd movement
• Symphony #9, 3rd movement

Chopin

• Raindrop Prelude

Debussy

• Girl with the Flaxen Hair
• Light of the Moon (Clair de Lune)

Grieg

• Notturno in C Major

Massenet

• Meditation from Thais

Mendelssohn

• Violin Concerto in e minor, 2nd movement

Mozart

• "Ah, I feel it, it is vanished" from The Magic Flute
• "O Isis and Osiris" from The Magic Flute
• Piano Concerto No. 21, 2nd movement

Tchaikovsky

• Pas de Deux from The Nutcracker
• March Slave
• Waltz from Sleeping Beauty
• Waltz of the Flowers from The Nutcracker