I found two excellent websites that offer FREE Buddhist ebooks (as pdf's). The first is the Numata Center, publishers of Gudo Nishijima's translation of Dogen's masterpiece, Shobogenzo. This title, amongst a handful of sutras (the Vimalakirti and Lotus Sutras, Sutra of Perfect Enlightenment, and the Hui Neng's Platform Sutra, to name a few) is available for free download. Click here to check it out.
Another great site is Buddhanet.net. For an extensive list of free ebooks as pdf files, click here. For general resources, ranging in everything from Buddhist comic strips, audiophiles, to an online magazine, click here.
As these are free resources, please support them with a donation if you can. If you know of any other open source resources, please feel free to share them below.
Happy reading and tons of metta.