Introduction to Economic Analysis, by R. Preston McAfee (2007). ‘Open source’ intermediate undergraduate microeconomics textbook. Website also has slides for teaching.
Introduction to Modern Economic Growth, by Daron Acemoglu (2007, draft). Advanced economic growth theory.
The Econometrics of Macroeconomic Growth, by Steven Durlauf, Paul Johnson and Jonathan Temple (2004, draft). Advanced growth textbook.
Environment Economics, by Ross McKitrick (2007, draft). Intermediate environmental economics textbook.

You can buy textbooks online for cheap at http://www.cheapesttextbooks.com
Amazing, this is great. I’m a high school senior, and I decided that I was …um, going to spend this summer teaching myself econ.
So thanks!
Very interesting. For commercial textbooks, check out the PhdEconomics website.