Homeschool Share has unit studies for ages 2 through 12. You will also find themes here.
Homeschool Helper Online has a nice selection of unit studies for various ages/grades.
Amanda Bennett has been putting together unit studies for years. Purchase one week or four week units.
Stone Soup Homeschool has a vast collection of links for unit studies.
Unit Studies listed by topic no mention of age/grade.
Easy Fun School has a nice selection of units with little to no mention of age/grade.
Elementary Tools 2000 has a variety of thematic units for grades K-2.
Houghton Mifflin Harcourt has a nice selection of popular themes for through out the year K-8.