What are some helpful websites and/or apps that are little-known to general public?

[young4ce_01]

I have previously discovered some websites that I use now daily or weekly, like:
http://sci-hub.se - unlocks access to most scientific articles <website>
http://libgen.is - pirated book library <website>
https://windirstat.net/ - shows memory used (visualized in graphic blocks)
https://cracked.to/ - this is some really cool website I found some time ago, you should check it out for sure