Other
Beloit College: K 12 Geology
This is a good source of basic information about density currents in water. It includes simple activities you can do to demonstrate these currents.
US Navy
Office of Naval Research: Oceans in Motion Currents
Use this site to learn what the Navy has to say about ocean currents and then test your knowledge with their quiz.
Other
Lunar and Planetary Institute: Oceans From Space
This collection of photographs from space-shuttle missions shows some features of oceanic circulation that simply cannot be seen from any other vantage point. This site is a great resource for many different topics of study.
University of Illinois
University of Illinois Urbana Champaign: The Heart of the Matter
A good quick look at the comparative anatomy of the heart of a typical bird and the human heart. Gives some of the facts behind the differences.