I have always liked the ELM website. I'm excited to learn that you don't have to download an app to use it on a mobile device - cool!
I used ELM a lot both as a student and when I was a classroom teacher. As a librarian, I find that I'm using less now for personal use but more that I'm recommending it and explaining to patrons - especially teachers and students working on research projects. It's just such a rich resource.
Our library does have a mobile app that we can use to access the catalog, place requests, etc. It's offered through Boopsie. I'm not crazy about it because I keep finding that it only searches our library and if a book is checked out it doesn't show up in the search results. I'm not sure why it doesn't work the same as our OPAC in that it should search all SELCO libraries and show if we have the book, even if it's checked out. I should be able to put a request on an item that is checked out. Perhaps I'm missing something...
We also use Zinio and Overdrive here as well.