Study Tools Online
Strong's concordance: You can put in a key word, and it will pull up all the verses with the word in it.
Jamieson-Fausset-Brown Commentary: One of my favorite Bible commentaries.
Easton's Bible Dictionary: A classic.
Smith's Bible Dictionary: Another classic.
Biblestudytools.com: This link contains several good commentaries.
Biblos.com: A HUGE site containing several Bible dictionaries, a Bible Encyclopedia, three concordances, and much more.