Hyperreal numbers: infinities and infinitesimals - "In 1976, Jerome Keisler, a student of the famous logician Tarski, published this elementary textbook that teaches calculus using hyperreal numbers. Now it's free, with a Creative Commons copyright!" (pdf—25mb :) [more inside]
The Hacker Shelf is nice crowd-sourced guide to (legally) free books on various computational and mathematical subjects. The topics page gives you an idea of the breadth of material available.
"Do you like fiction and mathematics? Are you interested in what our society thinks about mathematicians?" [more inside]
The Mathematical Fiction Homepage is a collaborative attempt to "collect information about all significant references to mathematics in fiction." Feel free to add classic or recent works in any medium to the collection, or rate existing entries on their mathematical content and literary quality.
Recreational mathematics and fractal graphics continue to stimulate the mind and foster student interest in mathematics. Some favorite authors & books in this area include: Martin Gardner's books (like The Colossal Book of Mathematics and The Night is Large), Cliff Pickover's books (like The Mathematics of Oz and The Zen of Magic Squares), Calvin Clawson's Mathematical Mysteries, Ian Stewart's books and puzzles, and Ivars Peterson's writings (like Islands of Truth). What are your favorite books and web sites in this area for stretching the mind and eye?