Shmoop.com isn't the easiest website to explain. It has so much great content that it's hard to know where to start.
Free stuff you'll find on the website -
1. Study Guides for ELA, Math, Science, Social Studies, Art & Music and more.
2. College & Career Help
3. Over 3,000 videos
4. Writing Labs
5. Teacher Guides
They also have online courses, test prep materials for SAT, ACT, PSAT and AP courses - these aren't free.
Shmoop aims to combine a teen-friendly, approachable style with academically rigorous materials. Here's a link to their video explaining a little more about their site - What is Shmoop?