It's not too late to take advantage of the Crossway/ESV Sale. Until 01/15, ebooks at Crossway and modules for ESVBible.org are on sale for $5.99.
I was originally particularly excited about the ESV Study Bible and GreekTools modules. I think the Study Bible module is definitely worth the price. While the GreekTools has a lot of interesting functionality, especially the interlinear options, it is unfortunately display only; you can't copy the Greek text, making it a disappointment.
An interesting alternative in the original languages is is the NA27-ESV parallel e-book. I was a week early, so I paid $9.99 for my copy. But it installs nicely into Apple iBooks. It displays correctly, but so far won't export it fully (there's a diacritics problem with the file, not the program, I believe).
I took the time to go through all of Crossway's ebooks, because for $5.99 you get both an epub and a mobi version of the book, and both are unecrypted. There are even a few that include a PDF version.
I think this is a complete list (the site doesn't appear to have a way to just list ebooks; this is a close approximation). Titles that caught my eye (and plundered my wallet) are listed below.
The Most Important Thing You'll Ever Study: A Survey of the Bible
by Starr Meade
The Theology of B. B. Warfield: A Systematic Summary by Fred G. Zaspel
Preaching Christ in All of Scripture by Edmund P. Clowney
God's Glory in Salvation through Judgment: A Biblical Theology by James M. Hamilton, Jr.