I had (very) recently started homeschooling my teenaged son, who's very bright.
I'm currently attending college, and I still have my textbooks from last semester. I use my world history and music appreciation books to teach him about those subjects.
Believe it or not, despite its unorthodoxy, it can actually work out great for highschoolers. Most college textbooks, especially in those subjects, are easy for teens to read and grasp. For those who can't, it's a good idea to paraphrase the books. Recently, I had done this for a friend who is experiencing a hard time financially (I paraphrased the history book because it's dry, and I typed it out and mailed it to her).
Plus, I'm pretty sure that high school AP courses offer similar books so this isn't very far-fetched.
For those who would like to give this a try, please feel free to either contact me or leave a comment to this message. I have my textbooks from last semester and this semester and am willing to help in any way I can.
On an unrelated note, I'd like to give a huge thanks to pmegan for that fantastic list of resources! How I wish I found this community first--it would have saved me a lot of headaches!