I recently came across AltStore via Twitter totally by accident even though after a simple search afterwards provided a multitude of posts and news about it's upcoming release... I must have been having a bad Twitter day.
AltStore is basically an alternative app store for iOS and whats interesting here is the was they have acheived it. There is no jailbreaks, no hacking. You simple install an app on your home Mac/PC and as long as you have Wi-Fi syncing enabled within iTunes you can use AltServer to install an app directly onto your iPhone.
The Technical Stuff
You're probably thinking how on earth has Apple allowed this to happen, well it works by using a well known developer feature that allows you to 'sideload' apps locally onto your device for testing purposes. The great thing about AltStore is you don't even have to be a developer, the software will install the app using a free Apple developer account, all you need to do is provide your AppleID and password to login and the software takes care of the rest. I know what you are thinking, there is no way I'm giving my details over to some shady app with the promise of the holy grail of AppStores...no need to worry as you can provide a fully revokable 'app-specific password' for the task.
There is a caveat to all this though, the 'side loaded' apps will only work on your device for 7 days due to the way the developer certificates works, the developers however have thought of a clever solution to help resolve this issue. As long as the AltServer sofware runing on your Mac/PC is on he same network as your iPhone the app will 'refresh' over the next week automatically keeping AltStore valid and working.
You're also thinking 'yeah but Apple will just shut this down' and they may well try but AltStore is using a method, which albeit is not being used for its intended purpose, that is used throughout the world by developers to test their apps before releasing to the Apple AppStore. Plugging this loophole would affect all developers who rely on it for testing causing repercussions throughout the developer community and thats something I'm not sure Apple can risk.
What's available and when?
Currently there is just one installable app, Delta Lite an excellent looking and running Nintendo emulator, but there is promise of more apps soon with a sneak peak of a
clipboard manager called 'Clips' and another Nintendo emulator capable of emulating the N64 amonst others.
AltStore is in preview at the moment (theres a Patreon membership to get early access and support the developer) but its due to fully launch on Saturday 28th September 2019 so check out the website and get ready for something special.
UPDATE 2: AltStore is now live!