Tool belt

Additional resources:

The NIV Study Bible: the study notes here are not perfect, but they are extensive, and nothing else quite like it has been done ; basically summarizing the best available commentary on each book of the Bible. The notes have been adapted to different Bibles, and even translations (For example, you can but the New American Standard Version with the NIV Study Bible Notes, a great combo. The one I would recommend, though, is the TNIV Study Bible.

Interlinear:
NASB-NIV Parallel New Testament in Greek and English (Marshall)... Extensive preview here

Many online

The IVP Bible Background Commentary: Vital for grasping the historical/cultural significance of a passage. Extensive OT preview here, Extensive NT preview here
Eerdman's Handbook of the Bible: It is amazing how this overview can sometimes nail the understanding/message of a particular book of the Bible, theme, or historical question. Full of charts and maps.
Small preview here

The New Bible Dictionary: The best one-volume Bible Dictionary; essential.

Concordance: Strong's (free online) and Young's are still popular, but since we don't circulate in the King James much anymore (and these are not "exhaustive or complete), The NIV Exhaustive Concordance is to be preferred.

Commentaries:
In order from most detailed/academic/use of Greek and Hebrew
todown-home, practical, devotional,sermon-oriented and "popular":

• New International Greek Testament Commentary
• Word Biblical Comemtary

• New International Commentary
• Inter Varsity Press Commentary (many volumes online free here at biblegateway.com
  
• Matthew Henry's Commentary (free here; concise version here)
  
• Warren Wiersbe's "BE" series

A good middle ground would be to start with the 2nd or 3rd above, and branch up or down from there. Individual commentaries within a series vary in quality, as each volume is by a different author, ideally a specialist in that book; but tha above series are so good, that most volumes are excellent. It would still be best not to buy a complete series, but pick and choose the classic volumes among 1, 2. and 3. One suggested list here

Word Studies:

• "Theological Wordbook of the Old Testament" (Harris, Archer, Waltke)..preview here/"Theological Dictionary of the NT" (Kittel)..note this second is many volumes, but helpfully abridged and summarized on one volume , nicknamed the Little Kittel. most of it readable online here
  
• The New International Dictionary of New Testament Theology, ed. Brown (overall most find this more helpful an accessible than #1)
  
• Vine's Expository Dictionary of New Testament Words

Online:

• BibleGateway.com
• Bible StudyTools.com (part of Crosswalk.com)