Diawi

Diawi (http://diawi.com) is a website where developers can upload their .ipa and .apk files (the compiled app) and share the links with anybody, so the app can be downloaded and installed on any iOS or Android device connected to the internet. The process is simple: 

1. Build the .ipa (iOS) / .apk (Android) in XCode/Android studio.
2. Drag and drop the generated .ipa/.apk file into Diawi's site.
3. Share the link created by Diawi with the list of testers by email or any other method.

Links are private and can be password protected for those apps with the higher need of security. The main downside is the management of the testing devices, as once the links are distributed, Diawi loses control over them, so the developer cannot know which versions were downloaded and tested. If managing the list of testers manually is an option, Diawi is a good alternative to Testflight.