What are some alternatives?
When comparing Remember The Milk and Vikunja, you can also consider the following products
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
Wekan - The open-source Trello-like kanban (built with Meteor)
Toodledo - Toodledo provides a set of tools to increase the productivity.
Google Tasks - to-do list app that integrates with Gmail and Google Calendar