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Updated
Wunderlist is a beautifully designed and simple to use to-do / task app you can use on multiple platforms including Android, iOS and Mac OSX. It is free for everyone, you can unlock additional features with a Pro-plan.
Specs
Pros
Cons
Con Acquired by Microsoft, So Future is Uncertain
Microsoft acquired Wunderlist in 2015, and therefore its future is uncertain. It's unlikely Wunderlist will continue to be an (1) independent product with (2) a free version and (3) continue to develop and innovate at a reasonable pace.
More likely, Microsoft will at least monetize Wunderlist the way it monetizes its other products. It's also likely that Microsoft will integrate it into its suite of office products and even its Windows operating systems.
To see what this is like search the Web for "Microsoft OneNote Mac OS X," and you will find a host of "standard" features only available in the Windows version, "official" documentation and tutorials that presume you have the Windows version (but never say so), etc.
One would also expect versions running on Linux to be worse or nonexistent. Also, Microsoft software in general tends to be bloated with marketable features but little concern for efficiency regarding memory usage, processing efficiency, or unobtrusive updates.
Con Will be discontinued
Wunderlist was purchased by Microsoft in 2015, and will be discontinued after the release of Microsoft To-Do.
https://blogs.office.com/2017/04/19/introducing-microsoft-to-do-now-available-in-preview/