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.
2
Google Keep

Save your thoughts, wherever you are

It's not exactly a to do app, but it's a great tool available on mobile, desktop and as a browser extension.
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