DK Find Out! is a new, free, online encyclopedia that I just learned about from Larry Ferlazzo (a great blog to follow, especially for ELL teachers).
There is some excellent content on here that is easy to understand. They are also continuing to add more content and promise to include more resources for educators, including links to curriculum and lesson plan ideas.
You can do a search or click on different subject areas, such as Animals and Nature, Dinosaurs, Earth, History, Science and more.
It also has quizzes, videos, and more and a "My Stuff" section being developed to bookmark your favorite resources.
The content is not in-depth and seems suited for early grades, but is well done.
Each entry has a description, image and other links. One feature I like is the related content links and the menu at the bottom showing where the current article fits. The labels on images also have pop-up windows with more information.
Take a look and sign up for more info as they release it.