I read recently in a discussion on IDEs over on Hacker News that rather than using a particular piece of software, one engineer considered the entire Unix system as his IDE. This idea totally blew my mind.
Since then I've installed Linux and started using it as my daily driver, gotten set up with i3 (which I've tried to use in the past and gave up on), revamped my Vim config, and have really started to make use of all its different features a lot more.
One particularly cool tool I just found through a bit of Googling is the
cal command. It'll output the current calendar – useful for calculating dates and things.