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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Wunderlist

Capture ideas, things to do and places to see

ThanasisUI/UX @travelplanet24 @airtickets_gr · Written
Simple and cross-platform!
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