University of Arizona
Cell Signaling
Superior site for information on cell signaling. Information provided as a tutorial with multiple choice questions to guide the learner. The site has lots of images and diagrams.
Khan Academy
Khan Academy: Cell Cell Junctions
Learn about the different types of intercellular junctions, including plasmodesmata, tight junctions, gap junctions, and desmosomes.
Biology 4 kids
Biology4 Kids: Connection and Communication
Take a look at different forms of cellular communication which takes place between cells.