Check out Scholastic! This might just be the best resource I've found online yet. And that's saying a lot.
The math mysteries are awesome.
The "writing with writers workshop" is incredible--some of the best writers teaching writing in their specialties (like Jane Yolen teaching writing myths). And your student can publish online at the end of the process. (http://teacher.scholastic.com/writewit/index.htm)
You should go see. I could practically homeschool my kids from this site alone.