For the past few years I have been working on an open source textbook. It contains material on calculus, functions of a complex variable, ordinary differential equations, partial differential equations and the calculus of variations. The text is still under development, but I believe that the current version will be useful for students and instructors.
I am trying to distribute the text to as wide an audience as possible, so please tell all your friends and link to my main page.
By ``open source'' I mean that all of the source used in the creation of the text is available for download. The LATEX source which comprises the text, the Mathematica notebooks and xfig files used to make the graphics, etc. are all available. I hope this will save instructors time, as they will be able to cut and paste portions of the text to make problem sets, solution sets and handouts.
Visit my links page for mathematical resources on the web. There are a number of sites that have free or open source textbooks.
I am writing the book in LATEX, the coolest of all typesetting systems.