Here are some other online to-do list apps :
1. Wunderlist :
It is a multi-platform to-do list app. It is available on iPhone, iPad, Apple Watch, Mac, Android, Windows Phone, Windows PC, Linux and even on the Web. It has over 13 million users worldwide.
HabitRPG is a fun online to-do list app. Actually it is a video game, it turns all your tasks into monsters. You have to defeat them to progress in the game. It has been featured in Forbes, New York Times, LifeHacker and few other publications.
It is the most voted to-do list on LifeHacker. It works on Android, iPhone, Mac and web. With this app, you can sync your to-do lists on different devices.
Others : Google Keep and Toodledo are also good to-do list apps.