The de Fesch (or just Fesch) Sonata in G Major is a relatively common baroque piece for double bass. I have the IMC/Sankey edition, and I find that a good alternative to Marcello. It's also one you'll find on a few bass syllabus' around the place.
There's a couple in Zimmerman's 'Solos for the Double Bass' - Evaristo F. dall' Abaco's 'Grave' is very nice. There's an edition of Vivaldi's Concerto for Violin in A major (1st movement) which is, well, Vivaldi. And then there's a Prelude and Allegro by Jean Francois d'Andrieu, a Handel sonata (C minor, originally for oboe). Though not all sonatas, they are worth taking a look at.
Michael is right - oboe and recorder pieces work well on the double bass, not only in range and melody, but the 'baroque' articulation on these instruments is easier to pull off on the bass, in my experience.