The stb library (Sean’s Tool Box) is a nice library with a lot of basic essential tools to make C programming more agile. There you can find all the other stb_* libraries that add up to stb.h, providing multimedia IO and more. They are all single file libraries and in public domain: easy to include in your project, and to tweak as you like .
SDL2 lets you get access to basic multimedia functionalities in a portable manner. You can create windows, use input devices, output audio, dynamically load code, work with files, create GPU contexts and render something. This library is thought for games, but you could use it for other things, for example VLC can output video using SDL. It is written in C and can be used in C++. Look at the wiki for documentation.
Rapidjson is the fastest C++ json loading library. It's pretty simple to use. You could use it to save and load custom data in your program.
Bootstrap is a well known and vastly used framework that lets you build up websites with cool appearance without being a html/js/css genius. It’s based on grids and has a cool navbar.
If you want to make use of grids in your website, but you want more than Bootstrap you have a lot of choices:
Clipboard.js is a cool and simple library that lets you copy text into clipboard without the hacker-hole Flash plugin. We use it in the rules section!
Primer is a CSS toolkit made by GitHub for his sites. It has cool and light features like the CSS tooltip you see if you click on the clipboard.js button in the contract in the Rules section.
Highlight.js make your code on the web full of color, like in your editor.
Detect mobile browsers is a small library that enables you to recognise if your site is being rendered on a mobile phone.