WordPress is one of the most if not the most popular platform to write in the web.
Mobile is growing fast and that also is for writers who would like to write without have to be in front of the computer.
What do I need?
For WordPress nothing more than one of their mobile apps, so far they have iOS and Android versions, you can check and download them here
The apps not just for write, but also you can check comments made on any of the sites you are author or admin, you can check statistics from Jetpack (if you use it) and read blog from WordPress.com you follow. Learn More