One of the things that makes Boundless interesting (and controversial) is the way it remixes OER materials into "alignments" with popular textbooks from commercial publishers. Rather than simply downloading an OER textbook, students can have it rearranged in such a fashion that they can download and use it in place of the more expensive text their professor actually assigned, knowing that it covers approximately the same material. The modular organization of other OER resources, such as Connexions, helps make this possible.
Boundless is organized as a for-profit company, albeit with a business model yet to be defined.
Sources: This catalog of resources and directories of resources is drawn from several other roundups, including the Free to Learn Guide on the Creative Commons website, Berkeley's guide to Open Educational Resources in Higher Education and the UMass Amherst Libraries OER guide.