I could make a huge list of apps that filmmakers might be interested in but someone has already done an awesome job at that!
These are my personal recommendations:
Shot Designer by Hollywood Camera Work
I have found Hollywood Camera Works method of designing a scene to be invaluable in helping me to visualize the blocking in a scene. I used to do it by hand by cutting out pieces of cardstock to represent characters, furniture, and cameras and moving them around to visualize the scene. Fortunately HCW has come out with and app that you can do this with on your phone, tablet, and computer. Worth every penny.
I have a cheap director’s viewfinder that I use but I also have this app which is great because it mimics what you see when you use different lenses. Handy dandy app if you ask me!