www.hartnell.edu/tutorials/ (nice interactive animation at the end of the page, good for self-testing)
www.biology4kids.com (very basic intro into cells, with a useful overview button on RHS)
www.wiley.com/ (nice animated tutorial)
www.shmoop.com/ (good details about eukaryotic cells)
Get into medicine – 600 UKCAT Practice Questions – Oliver Picard et al.