Olivetree.com: An online 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.

Biblestudytools.com: This link contains several good commentaries.

Biblos.com: A HUGE site containing several Bible dictionaries, a Bible Encyclopedia, three concordances (including Strong's which I consider the best), and much more.