Excerpts from the evaluation criteria section:

To gain our seal of approval an open source mathematics textbook must be able to serve as the primary text in a mainstream mathematics course at the undergraduate level in U.S. colleges and universities.

We are looking for books that we can recommend for consideration to any of our colleagues, books that are suitable for use in traditional university courses. Books must be mathematically sound and written in standard English with evidence that they have been proofread and edited. They need not have color, flashy graphics, or ancillary materials. They must have exercises, and though it would be desirable to have short answers to many of the problems, it is not necessary for approval. It would also be desirable to have more complete solutions available—at least to an instructor.

We do not require any particular form of license, although the Gnu Free Documentation License and the Creative Common licenses (in its many varieties) are what we recommend. These licenses allow the book user the greatest flexibility in making use of the material. We will approve a book with copyright restrictions as long as the book is available at little or no cost.

TL;DR: Free (or really, really, cheap) books for mainstream undergraduate mathematics audited by AIM.

kantos - take a look at A Gentle Introduction to the Art of Mathematics for your proofs class! How's it going by the way?