GetBodySmart is a rather nice online textbook on human anatomy and physiology, and unlike the Environmental Science “course” that’s more of a textbook, this ‘textbook’ includes a number of activities, animations, etc. It also apparently has a special iPad version.
The American Society for Cell Biology has various biology education resources http://www.ibiology.org/
You can watch a range of animations explaining various biology topics at http://www.sumanasinc.com/webcontent/animations/biology.html
The Howard Hughes Medical Institute has a number of resources on a wide variety of biological science topics (from neuroscience to obesity), aimed at both younger and college students.
The National Institute of General Medical Education has some nice resources on biology topics, including downloadable booklets and interactive quizzes. https://www.nigms.nih.gov/Education/
Harvard University has a useful resource for first-year students entering Life Sciences courses. To help students refresh or learn necessary concepts that they should have learned at high school, CyberBridge has tutorials and assessment quizzes on various basic topics. http://cyberbridge.mcb.harvard.edu/
The University of Maryland has done a nice job with their MathBench Biology Modules. These modules introduce students to the math underlying intro biology courses, in as interesting and entertaining way as they can contrive. The modules contain hundreds of interactive activities, games, and questions. https://mathbench.umd.edu/
Molecular Logic has activities for what it calls "Molecular Stepping Stones" — 10 physical-chemical principles that underlie many biological processes. http://mw.concord.org/modeler/
Science at a Distance: E-Learning Modules offers online learning modules on a variety of biological topics, such as the properties of carbohydrates and Mendelian genetics. http://www.brooklyn.cuny.edu/bc/ahp/Modules/Modules_HP.html
Designed to supplement a biology textbook, Biology: Online Labs has 31 virtual labs that recreate actual scientific experiments in a raft of biology topics, from why tropical songbirds lay fewer eggs to how regulatory genes govern vertebrate development.
Microbial Life has a variety of educational resources available for students and teachers, both at the K-12 level, and for undergraduates.
a nice site on amino acids
a "hypertextbook" on the subject of pathophysiology of the digestive system from Colorado State University
Cells alive! has many interactive animations to help you or your students understand how cells work. https://www.cellsalive.com/