As of right now, Android is my favourite mobile operating system for one main reason. It is open enough and legal for developers to customize and create their own flavours of Android. With certain limitations, it is even possible to use a custom forked version of Android in a commercial product, as Amazon.com has done with the Fire OS. Although this creates further competition for Google, openess has vaulted Android into the number one mobile OS spot in terms of popularity.
Depending on your device's model, there are many different custom Android versions (ROMs) available to download. Popular ones include
Samsung Galaxy S2 HD LTE (SGH-I757M)
For example, if you have the Samsung Galaxy S2 HD LTE (Model SGH-I757M), @titanic_fanatic from xda-developers.com has customized his own version of the Carbon ROM
for this model. Based on technology from Carbon, CyanogenMod, SlimRoms, AOKP and Paranoid Android, you can get a special flavour of the Android KitKat 4.4.x OS for this device.
ODIN & Recovery Manager
For Samsung device owners, you can use ODIN on Windows to flash your smartphone and install a recovery manager. Doing so will allow you to easily wipe your phone and install a custom Android ROM. Created by Samsung and subsequently leaked to the public, ODIN
is software that allows one to flash firmware on their Samsung device. Remember to check the legality in your region before doing so. A popular recovery manager is PhilZ Touch Recovery
(CWM Advanced Edition) by @Phil3759 that is based on ClockworkMod