Lulu.com is a book distributor, which means it will make your book available through the websites of Amazon, Barnes & Noble, and Apple’s iBookstore. Each of those distributors takes a big cut of your revenue (roughly half). For books, ISBNs are obtained through Lulu, and LCCNs are granted by the Library of Congress.
Lulu has its own online marketplace where people can purchase your book. ETC Press only links to Lulu because the revenues on those books are substantially higher for authors. (This is how many independent and small publisher operate.)
Lulu also lists the book in the bibliographic databases of Ingram, Nielsen, Bowker and Google Books, which enables any library or bookseller to order copies as well as people to find ETC Press books through internet searches.
The free downloads are hosted in CMU Libraries digital repository, which provides publications with Document Object Identifiers.