National Geographic's Traveler's Guide to the Planets is a wonderful, fun, visually spectacular resource for students learning about our solar system and the planets. When you first arrive at the site, it has a stunning, exciting opening animation and then it gets to the main page.
The main page, seen above, has two panels. The left side panel allows you to select which planet you want to learn about (Pluto is still listed, Earth is not since we live there) and what you want to know about each planet. The site is set up as a true traveler's guide and includes info such as history, trivia, sites, advice, climate, and luggage. This is a great way to get students to learn about the planets in a new way.
I had a lot of fun going through the site and learned some new things about the planets. It is appropriate for grades 5-12 (and maybe even younger with some teacher help).