I am not aware of any free apps that are really good, but I have used the Shooter app with reliable results and it goes for about $10 if I remember right. Like any other ballistic computer, it's only as good as the data you put into it, IE knowing your weapon and bullet specs is the key to getting accurate results.
2 best for long range stuff (may not have reticles to pre load) are Hornady 4DOF and Applied Ballistics. Hornady is free- bullet and powder selections are growing but haven’t quite reached what Applied Ballistics has. it’s also used by Mile High Training Detachment and a bunch of Snipers Hide dudes so endusers are testing it and pushing it to its limits.
Applied Ballistics costs a bit and there are some questions about the truing methods they use in transonic and subsonic realms but it has a solid bullet selection and is well used/supported.