I have recently discovered the free Bible study tool called e-Sword. This tool is very nicely done, and there is just a wealth of resources on the internet for it. My library is up to over 1GB of books at absolutely no cost.
I started out using Online Bible, but was irritated by it for one reason or another. So then I purchased the entry level Logos software, but am disappointed at the lack of affordable resources for it – given I am not in the ministry and cannot shell out a couple thousand dollars on it.