I started using Trello. Some ex co-workers introduced it to me a few years ago, but i started using it daily for only half a year now.
I find it verry nice, with a simple and clean interface, options to create boards / lists / cards, options for checklists, due dates, label coloring, attachments and all sorts of useful things. It's also colaborative, you can create teams and give access to other people. They can add comments and work together.
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