You should probably know about... (17 Part Series)
Curl, really? Well, I'm running out of topics that might not be that known, so I thought I'd recommend something that probably a large majority knows about. And I'm also a bit proud to come from the same country as the creator of curl, Daniel Stenberg. And finally, if there are junior developers reading this, it is possible that you never heard about curl before.
Curl is a network protocol tool. You can make all kinds of requests with curl, http, ftp, pop3, imap and the not so much used anymore Gopher. It's basically a swiss army knife for protocols.
So if you don't already have it installed already, go download it and start trying it out. You will find it useful.