HippoCampus has free biology classes online. You can choose a textbook, and then watch an audiovisual presentation about each topic. The animations make the lessons very interesting.
The University of California, Berkeley has videos of its General Biology course online.
Apologia Biology Vocabulary - This page has crossword puzzles and flashcards for vocabulary words in Apologia Education Ministries' Exploring Creation with Biology (2nd edition) by Dr. Jay Wile and Marilyn F. Durnell
Interactive cell structure - Wiley has an interactive unit that teaches about prokaryote, animal, and plant cells.
NeoK12 has short instructional videos about cells online.
Cells Alive! has interactive models of cells.
Nancy Clark, a retired science teacher, has a page on her website about cells which includes experiments, activities, crossword puzzles, and lots of other links about cells.
Crayola has a coloring page for labeling the parts of a cell.
National Geographic has interesting videos and articles online about many different animals.
Enchanted Learning has pictures to color and label, articles to read, forms for making reports, and many other resources. You can access the website for free, but if you pay the $20 yearly subscription fee, you can print pages without ads.
All About Birds is an online guide to birds and bird watching that is offered by Cornell University. This site is packed with information about birds! If you look up a bird, you can also listen to a recording of a typical bird song for that type of bird.
Resources for a bird unit study including teaching points about birds, a printable list of children's books about birds, a bird matching game, and a printable reading log for books about birds
Facts about Ants - Teaching Points about Ants, Children's Books about Ants, Reading Logs for Books about Ants and Insects, and Links to Other Resources
The Arbor Day Foundation has an interactive website for learning about trees.
Scholastic.com has a unit plan about plants. There is a link to a Seed Growth Recording Sheet on this page. There is an outline of a pot for six days, and children can draw a picture of what their plant looks like each day.
BBC has a game for learning the parts of a flower. You click on each part of the flower, and drag it into the box with the correct name on it.
Beyond Penguins and Polar Bears has links to activities and lessons about plants
Crossword puzzles and vocabulary flashcards for Apologia Educational Ministries' The Human Body: Fearfully and Wonderfully Made! by Dr. Jay L. Wile and Marilyn M. Shannon, M.A.
GetBodySmart.com has interactive animations for learning about the human body.
Vitamins and Minerals - The Open University has free online course about vitamins and minerals.
The Digestive System - This is a link to a Quia game about the digestive system. Match the name of the digestive organ to the correct number on the picture.
Crossword puzzle for Joanna Cole's The Magic School Bus Inside the Human BodyQuizlet flashcards for learning the parts of the ear