KaiOS is described as 'brings smartphone-like functionalities to affordable phones: the KaiStore for apps. Video calls over 4G. Mobile payments through NFC. Even dual-SIM support' and is an app in the OS & Utilities category. There are more than 25 alternatives to KaiOS for a variety of platforms, including Android, Linux, Self-Hosted solutions, iPhone and Android Tablet. The best alternative is Android, which is both free and Open Source. Other great apps like KaiOS are Ubuntu Touch (Free, Open Source), Replicant (Free, Open Source), Plasma Mobile (Free, Open Source) and iOS (Free).
Amazon Fire OS is an Android-based mobile operating system produced by Amazon for its Fire Phone and Kindle Fire range of tablets, and Fire TV; the tablet versions of the Kindle e-readers are the Fire range. It is forked from Android.
Project Sandcastle: Android for the iPhone Where sandboxes set limits and boundaries, sandcastles provide an opportunity to create something new from the limitless bounds of your imagination. Project Sandcastle is about building something new on the silicon of your hardware.