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I completely reverse-engineered a popular online task management system, but made it an application that runs locally on my laptop. Now I control all the functionality, and most importantly, the data. Also, I can add things that maybe wouldn't be interesting for the many, but are very useful to me (e.g. when I enter a URL as a task, it fetches the title and sets that as the task name; e.g. it auto-summarizes arxiv postings as discrete tasks and takes a crack at prioritizing them for me; e.g. all my email gets auto-funneled into separate tasks, so I'm always at "Inbox zero").
I would say it has improved my productivity and focus a lot.
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How about you, @kureikain?