The following pieces that are on this site are good examples of modulation.

Pieces that are not in Public Domain are marked NON-PD and may not have scores.

ClassicalDykes, John B.English (UK)Eternal Father, Strong to Save (Navy Hymn)Modulation, Secondary Dominants
RomanticHensel, FannyFemaleSix Lieder, Op. 7 No. 1: NachtwandererModulation, secondary dominants, Aug 6
RomanticLe Beau, Luise AdolphafemalePiano Quartet, Op. 28, Mvt. III: Tempo di MazurkaCompound ternary, modulation
RomanticLe Beau, Luise AdolphaFemalePiano Quartet, Op. 28, Mvt. 1: Adagio, Allegro con fuocoRondo, Aug 6, Neapolitan, Enharmonic modulation
RomanticSchubert, FranzAustrian, MaleMoments Musicaux in Ab Major, Op. 94 No. 6 D780Ternary – compound, rounded binary, Aug 6, cto7, Neapolitan, enharmonic respelling
ClassicalReichardt, LouisFemaleWeihnachten (SATB)modulation
Jazz, Pop, and Film MusicRodgers and HartAmerican“My Funny Valentine” (1937) – NON-PDmodulation from minor-rel major, Tritone sub