PhET Colorado Simulations is aimed towards college-level and high school biology students and includes interactive simulations on many different biology topics.
Khan Academy has many example videos of problems in biology. This is a great tool to brush up on your understanding of biology topics.
CELLS alive! captures videos and computer-enhanced images of cells covering the cell cycle, mitosis, meiosis. This site also has puzzles and quizzes for fun cell enrichment.
Root Onion Tip is another resource on looking at the cell cycle and mitosis.
Biology of Plants has good reviews of the plant life cycle with animations and videos.
Linnaean Kingdoms is a good chart of all the kingdoms and to identify differences between them.
Animal Diversity is an online database and encyclopedia of animal natural history and enables the exploration of biodiversity.
Regents Exam Prep has reviews, study-aids and problem sets in various biology topics.
Biology Resources is a great tool for biology students to brush up on biology skills and includes problem sets, videos, and worksheets.
Brightstorm has online biology video lessons to help students understand critical biology concepts so they can improve test and quiz scores and more easily complete homework assignments.