http://www.biblestudytools.com/ – (lately, this one seems to have fewer free items and more for sale) ￼very comprehensive. Commentaries, encyclopedias, and much much more.
http://biblehub.com/ – comprehensive. Many many commentaries. Has devotions. Can be a bit hard to use (but I have three tips below) because they use lots of 3-letter acronyms. Includes a nice parallel comparison of verses from various translations.