Extreme Science is a website that has a lot of simplistic definitions and explanations for many of the common themes and topics of the Earth Sciences and Biology.
This is more geard towards younger students like elementary age, however some of the definitions are a foundation to any math problem. Use them if your students are confused about a new lesson topic to help show them what something means.
This website allows the user to create their own comics. I think it would be really fun and definitely different to incorporate this tool into the classroom, perhaps by creating a comic in German and having students discuss it, or I can ask them questions about it. Or, they can create their own comic to show what they have learned. It's fun, creative and different.
A good site to show students. If they are at home (and have a computer) and don't understand some homework assignments, they can go to this site to get examples and definitions. In high school they can use it for Algebra, Calc I, and Calc II