I am a big fan of Logos 4 software digital library system. My library is available online, on the desktop, via mobile device anywhere and everywhere (PC or Mac). AMAZING APPLICATION! This is what it looks like:

As a software developer I have seen and tested many applications. This is one of the best applications I have ever used from a usability tester and from a user perspective. How everything is laid out, how it links to one another, how parallel resources can be switched using arrow keys, the list goes on. I have the older Logos 3 Scholar's Library: Gold package plus the Thomas Nelson collection and many other add-ons. If you are interested in bible study as a casual study or a theologian I recommend one of their base packages. You will not be disappointed I will assure you of that.