E-Sword Bible Software...or something similar. I love E-Sword because it's a great deal(lots of it is free) and once it's installed you don't have to have an internet connection to use it (ladies retreats and youth camps are often held in less-than-technofied locales!) I downloaded the free program (which included KJV w/Strong's, ESV and some basic reference tools) and then I bought the NLT version, a cool maps app and a full set of commentaries. E-Sword has a bit of a steep learning curve, but once I got the hang of it I was so glad to have it.
The Expositor's Bible Commentary. There are lots of versions of this, but the one I linked is concise and adequate for most basic research projects.Also, I found mine at www.half.com for a really great price.
Zondervan Bible Handbook. Such a great all-purpose, easy-to-use book. I even recommend this one for teenage girls who want to dig a little deeper into the background and historical context of the most beautiful Book in the world.
