I ran across the AIM math website recently by stumbling onto their section for free math textbooks which, according to their evaluation criteria: "...must be able to serve as the primary text in a mainstream mathematics course at the undergraduate level in U.S. colleges and universities....Furthermore, we expect books of the quality we seek to be class-tested. They should have been used (and be in current use) by faculty other than the author....We are impressed with books that have a support system, signs of which are a website that is maintained, a community of users, a means for submitting support material from faculty such as test questions and project ideas, and a mechanism for improving the book by correcting errors, publishing errata lists, and displaying user comments."
These are some of the best free textbooks out on the internet; you should take a look. While looking around the website I found the Math Teacher's Circle Network had some resources that you might find useful as well. The latest newsletter (Summer 2013) focuses on Common Core math and the Resources section has some PDFs you might be able to integrate into your classroom. For example, the "Introduction to Problem Solving", "Mathematical Games" and "Introduction to Matrices" PDFs stand out.