1. The ESV Bible – it is free and it is a newer translation.
2. The Brown-Driver-Briggs Hebrew Lexicon – this is a standard reference for Hebrew word studies and it works with the Strong’s numbering system as well.
3. Thayer’s Greek Lexicon – also a standard in Greek language study.  It is also keyed to Strong’s.
4. The International Standard Bible Encyclopedia (ISBE) – want to know more about a person, place, or thing in the Bible?  This is an excellent source arranged alphabetically by topic.
5. If you are a student of Greek or Hebrew, do not fail to download any or all of the original language texts available (a few are keyed to Strong’s for easy cross-referencing).
6. Institutes of the Christian Religion by John Calvin – a must for every library!
7. A good devotional is Spurgeon’s Morning and Evening.
8. Keil & Delitzsch’s Commentary on the Bible – it is older, but still excellent.
9. Barne’s Notes on the Bible – still has some value even if it is older.
10. The Jamieson, Fausset and Brown Commentary – another older commentary that still has some great value.
11. There are many atlases that are good if you want to study maps.
12. Also valuable are Nave’s Topical Bible and Torrey’s Topical Textbook if you want to study what Scripture says about a particular topic.

There are many other add-ons available for download.  You can go and decide for yourslef what will be helpful to you.  For a free program, I think e-Sword is tough to beat!  Let me know what you think if you already have it or decide to give it a try.