For the month of March, Crossway is granting free access to the ESV Study Bible online. Kick the tires here (free login required).
My favorite feature is the concluding “article” in the back of the Bible, so buried that is has probably been discovered only by me and a handful of people who got lost while trying to find the maps. I’m talking about the History of Salvation in the Old Testament. No student of the Old Testament should be without this list!