Study Tools Online
Strong's Concordance
You can put in a key word, and it will pull up all of the Bible verses that have that word in it.
Easton's Bible Dictionary
A classic Bible dictionary.
Biblestudytools.com
Contains several good commentaries.
JFB Commentary
One of my favorite Bible commentaries.
Smith's Bible Dictionary
Another classic Bible dictionary.
Biblehub.com
A HUGE site containing several Bible dictionaries, a Bible encyclopedia, three concordances, and much more.