We can actually achieve almost 80% of the things we do on our personal computer with our smartphones. Smartphones are mobile devices which users use for communicating, multimedia and accessing internet etc. Most mobile phones run on a single user operating system like IOS and Android. Mobile devices in 2019 come with enough system resources for better computing and processing of large data. Mobile devices now come with Artificial Intelligence integrated OS to help automate processes and increase the computing speed of our devices.

Android which is the best operating system for Mobile devices, comes with a simple user interface that's allows users to seamlessly multitask, because of its open source privileges the operating system is evolving rapidly. Every year android users get a new software upgrade which comes with tons of features and changes to help make things better.
There are a couple of tools you can use to start programming with your Android phone, I will only recommend the best I have used, you can still do your research and find more tools also don't limit your knowledge to the scope of this article alone.

For Web Developers
KSWEB

This application is actually worth it for PHP Developers. Some of the features include

• Lighttpd server v1.4.35
• Nginx v1.13.1
• Apache v2.4.28
• PHP v7.2.0
• MySQL v5.6.38
• Msmtp v1.6.1
• Web Interface v2.11
• KSWEBFTP v1.0
• Editor v1.1
• Scheduler

I still use this application to test my Laravel / PHP based project to check the responsiveness on mobile phones before deploying online.

http://www.kslabs.ru/

BITWEBSERVER

It all wrapped into one application called Bit Web Server. So easy to run on android device, just single click then you can run web applications with PHP scripts and MySQL database.

This application is specially designed for mobile and tablet, so this application requires very small memory.
Bit Web Server is also a web server app for android, some of its features include:

• LIGHTTPD as web server,
• PHP as PHP server,
• MYSQL as MySQL server,
• PHPMyAdmin as MySQL Client, and
• MSMTP as SMTP Client,

http://bws.andiunpam.com/

AnWriter / AnWriter Pro
This application is a text editor used to write HTML, JS, PHP and CSS, It also comes with an auto-complete and syntax highlighting feature just like Vscode and Sublime text on PC.

http://www.anwriter.com

Quickedit / Quickedit Pro

QuickEdit is fast, stable and full featured text editor for Android. It is optimized for both phone and tablet devices!

Features:

• Enhanced notepad application with numerous improvements.
• Code editor and syntax highlight for 40+ languages.
• High performance and real time feedback on large text files (more than 10,000 lines).
• Show and hide line numbers.
• Undo and redo modifications without limit.
• Quickly search and replace.
• Easily input color hex.
• Automatically detect character encoding.
• Auto indent for new lines.
• Preview html and markdown files.
• Be able to edit system files for rooted devices.
• Access files from FTP, Google Drive, Dropbox and OneDrive.

Android Developers
AIDE 
AIDE supports developing Java/Xml based Android apps using the Android SDK. The AIDE app comes bundeled with a mobile version of the Android SDK, so there is no need to install anything else. The following tutorials show how to get started developing Android SDK apps with AIDE. To learn more about the possibilities of the SDK visit developer.android.com

https://www.android-ide.com/

Sketchware.
A simple but powerful application that allows you to build complex android applications with your mobile phone. it comes with a drag and drop interface just like Android studio then you will be able to edit the contents and apply your programming logics, boom then watch your application compile with ease.
http://sketchware.io/

Java Developers
Java N-IDE
This project is develop to help the community learn java on android.

You can build and run Java file with JDK 1.7.

Features

• Offline java compiler (support jdk 1.5, 1.6. 1.7 or higher if your device support)
• Java auto complete when typing
• Build android app
• Build java library ✔
• Java code formatter (Google Java code formatter) ✔
• Build Android app. ✔
• Android Logcat.

Python Developers.
QPython3
QPython3 is a port of Python3 for android, it can run Python3 applications on your android devices like mobile or tablet, It also contains the Package Index where you can find many funny and usable Python applications easily.

Some other applications that also comes in handy include:
Es File Explorer/ Solid File Explorer
File managers that help you navigate on your phone easily.

Google Keep 
Best Notepad on Android you can store your notes on cloud, you can even gain access to your notes on your PC through Google keep website. This application is very useful for content writers.

Picsart 
Since android users can't get Adobe Illustrator, Corel draw or a standard Photoshop, Picsart is the best option for photo editing and logo creation.

Termux 
Have u ever wondered how to run shell commands on your Android phone. This application does all the magic for you.

NB: Every application listed above is currently available on the Google Play store.