What are some good cross-platform to-do applications?

I already know about all the popular ones like wunderlist and any.do etc. I like the look and feel of any.do, so anything like that. Free would be preferred, looking for an app that I can also use in Windows or through the browser.
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Dropbox Paper

Collaboration tool built for teams

Works beautifully across browsers, iPhone and Android phones. Create simple to-do lists for personal use, shared to-do lists with people from work, and the best part is, if required, you can discuss to-do items very easily with others. If you ever need it to, the to-do list can evolve into much more and become a full working document as well if you're planning something that requires more detail for example. I use individual Paper docs for each type of list. Here are some examples of docs I use for to-do lists: 1. This week's priorities (this is a shared one with the team) 2. Personal priorities 3. TV shows to watch… See more
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