Free History Books Online has a great list for history books online. I checked out some of the links and some include more than just history.
Heritage History has a great selection of non fiction titles for history includes different levels all around great site not just free books, timeline figures, maps and more.
The Puritan's Homeschool Curriculum various texts and more for an array of subjects excellent resource for Christian teaching. Seems geared towards upper level learning.
We Give Books has an extensive collection of books to read online from pre-school age through ten.
The We Do Listen Foundation has ten books online geared towards teaching children lessons on behavior, also includes lesson plans for each book, songs, and posters. There is also a section for playing games, and creating your own book.
The Library of America has a variety of free books to read online, or subscribe to story of the week to receive a new story each week. Great for high school students.