If you're doing life science with your middle schoolers this year, these sites can help:
Science-Class.net: Many free science resources for elementary and middle-school grades. The section on cells features activities and labs, slideshows, games and quizzes, and links to interactives and other resources.
Biology Corner: Another site with many lesson plans, worksheets and activities focusing on anatomy, biology and physics. The section on cells features lab activities, study guides, and interactive lessons.
Cells Alive: Interactive illustrations of animal, plant and bacteria cells. View the site's BioCams to see cells growing. Downloadable worksheets are also available.
Cell Biology Slideshow: 242 slides
Chapter on Cells: 24 pages
Cell Download: Instructional slides from a teacher's site featuring information on cell theory, cell cycle, cellular respiration, cell similarities, cell structure and function.