Ran across this today via the Chronicle for Higher Ed. Textbook Revolution is a collection of online (read free) textbooks for students. Apparently started by a student who was frustrated at paying so much for his class textbooks (what a corporate racket the market is) he decided to take matters into his own hands. The site is impressive in its breadth, although somewhat lacking in the Humanities (but in fairness we use fewer textbooks). At any rate it is probably worth checking out as a way to drive down the cost for your students.
And while we are on the subject Kudos to to Matt Burton whose Wiki book Rhetoric and Composition won the book of the month award. This one is a free Writing Textbook. (I wonder if this counts as a tenure book?) You can check out the larger collection of Wikitextbooks, again read free at Wikibooks.