The Freakonomics blog has a post about free books available online. The recent free release of a Suze Orman book was an example along with a few other ones.
The blog post in itself is interesting but the other resources pointed to in the comments make it even better — several other free book opportunities are noted including links to other lists.
If you’d like to explore how the world of the university press world is gettin’ free, try this search on Google or Yahoo along with your favorite keywords: “university press” free download site:edu . (I found doing a more general search like free book downloads or free ebooks brought a lot of junk.) You’ll get some irrelevant stuff, but you’ll also get pointers to small university presses which are making their books available online for free.
To get other general overviews of what’s available, try site:edu free book downloads “university press” oxford yale harvard. Though that search will give you a lot of table-of-contents downloads and portions of books. And too keep up with new options, try free books download “university press” inurl:2008 (The inurl: portion is because many blogs archive by date, and inurl:2008 is an easy way to find recent entries.)