Symbian OS

The platform was popular until 2010, after which it began to be replaced by Google's Android. Eventually, development stopped in mid-2014.

The graphics toolkit used by Symbian has always remained the native Series 60 (later AVKON). It can be easily controlled using a regular keyboard or a QWERTY mini keyboard.

The software was binary and compatible with most versions of Symbian, including the 3rd generation on the Qt platform. This framework has become the preferred user interface toolkit for developing new applications.

Pros and Cons
It does not load the CPU and RAM too much; you need to install additional software to communicate with the computer.
There is a memory release function;     
Good virus protection.