Hi--it's true that Bach wrote a lot of lighter, less heavy sounding pieces. Even some of the Preludes and Fugues, like those in G major and C major, are kind of light. Besides the composers mentioned by Giovanni and Acc, some of the more famous composers for organ include Widor (who wrote Symphonies for the organ--some movements are light, others are not), Franck (again, some of his pieces are lighter than others--the Prelude, Fugue and Variation; the Finale; the Pastorale; and the Piece Heroique are some lighter ones), Mendelssohn, and Buxtehude (he was one of Bach's models, and many of his pieces are quite lively.) There are lots of others, but these are just a few of the more famous ones.