Bible.org is one of those sites I use constantly, and now it has been completely redesigned and is easier to use than ever. It has so many helpful articles and Bible study tools that it would be hard to list them all here. I am especially fond of the way I can have it list all their articles according to the Scripture passage I am currently studying.
I also find the 'Pastoral Helps' (under 'Communities') section very worthwhile. It has a wealth of information, from various preaching and teaching aids for special occasions (such as weddings or funerals) and holidays to administrative and organizational aids. I also use their database of sermon illustrations (under 'Study Tools') on a regular basis. You can search for illustration by topic or Scripture passage. Long-time blog readers may remember that I recommended this feature of Bible.org in an old article entitled Online Sources for Sermon Illustrations. Well, I haven't changed my mind, it is still one of the best and easiest to use online resources available.