Like many others, I use the Olive Tree Bible Study app. I have it on my phone, my pad, and my desktop machine.
Not only does it provide access to many versions, but it also supports many dictionaries, commentaries, devotionals, and other aids.
I also use Kindle a lot for devotionals and aids. Many times a Kindle title is also available on Audible, and when I am particularly lazy, I will have the audio playing while the Kindle keeps up, highlighting the text as the book is read to me.
- YouVersion Bible app for when I'm on the go and other free versions of the Bible. It used to be much better than what it is now for their Public Note system, so that's why I only use it for personal use. Online and offline uses available.
- e-Sword app for when I'm taking notes during sermons. Very easy to use and the interface is just as well. You can quickly link Bible verses in the notes section and use it offline as well.
- FaithLife/Logos Study Bible app for well, as it states. It has more extensive features and it has courses. This one however can (the app is initially free though) be a little pricey depending what you want. This is why I have YouVersion so I don't have to spend money on versions I already use.