Windows: You might use the Windows clock/calendar on the task bar to quickly check what day it is on the fourth of July. What you might not know is that Windows can tell you this for every day from 1 January 1900 until 31 December 2099. That's two hundred years right there in the lower right corner of your screen!
It's fairly simple: if you click the clock, Windows shows you a nice little calendar. You can browse to previous or next months easily with the arrows on both sides of the month (or your scroll wheel). But I needed this information about 17 February 1967 and didn't feel like clicking until my finger fell off.
(Why is my date in Dutch and the rest of my OS in English? Ask Microsoft!) As it turns out, if you click the name of the month itself, the calendar zooms out to a whole year:
Click the year and it turns into a decade. Click the decade and you get a whole century:
You can switch between 20th and 21st century with the arrows, and then you can 'zoom in' again on your preferred month.
Maybe you've known about this for years, but I only discovered this just now because I needed to check that day in 1967 and I didn't want to open my browser. Of course, I have now opened my browser to write this article. So much for staying productive!