Monergism – 750 books in multiple formats (ePub, mobi, and PDF) for free download. Plus, dozens of free resources on the Bible, ministry and theology. All books are in English. Each of the multiple language websites below has English books as well.
Spurgeon Gems – the largest collection of Spurgeon resources online, including a 63 volume set of sermons. From Chapel Library.
Orthodox Presbyterian Publications – 24 free ebooks in ePub, mobi, and PDF.