VideoANT make it possible to annotate a web-based video
The University of Minnesota has developed a wonderful tool for annotating videos called VideoANT. The program allows individuals to place markers in a web-based video and then add comments. This is a great way for instructors to direct student attention to key points in a video, or to ask students to find key points and comment on them. The link to the annotated video can then be emailed, which would allow students to submit their annotations as an assignment.
It's extremely easy to use. When you first go to the site, you can watch and experiment with a demo video. It's a great way to become familiar with the program, and takes only a few minutes to master.
One tip: I would put the marker (or have students put their markers) at the beginning of a point of interest, rather than at the end or middle. The markers can be moved at any time.
There are a number of interactive and multimedia sites on the web that are useful for educational purposes, but aren't exactly "slick" looking--like those done by professional web masters. This site on cell biology is like that: http://www.johnkyrk.com/index.html
This site contains a number of animations designed to explain various aspects of cell biology. The topics include: cell membranes, glycolysis, Golgi apparatus, Mitochondria/electron transport (to name a few).
This site is much slicker than the one listed above:http://learn.genetics.utah.edu/content/begin/cells/ Click on "Inside a cell" and you'll be taken to the outside of a cell. Mouse over the cell, and you'll get a look inside. Click on an organ, and you'll get a slick video, complete with sound effects describing the organ and what it does.
Text, audio and video describe endoplasmic reticulum