I am a primarily a coder, and although I can implement design, websites and am creative from a technical perspective and have a eye for design, I don't really have the skills to originate graphic design, multimedia or user interface elements quickly whilst getting the development work done.
To help make things look good and professional, I have compiled a collection of useful design resources for developers who need some extra help with the design side of things.
Photographic images https://unsplash.com - Free images (and they are pretty good)
User interface (UI) / User Experience (UX) and for OG's: Graphics User Interface (GUIs)
https://getbootstrap.com - UI design / CSS Framework
https://www.font-awsome.com - Web Icons
Graphic elements / images
https://trianglify.io - Low-poly style background (or a free coded version here: http://jsfiddle.net/brightertools/69Ljdusc/10)
https://undraw.co/illustrations - Free illustrations (with colour selection/modification) http://www.xiconeditor.com/ - Create website icons (favicon.ico)
Graphics Applications
https://www.getpaint.net/index.html - Image Manipulation application
(although I use also Adobe CC, this tools is great for quick for image resizing and editing)
Content Management Systems / Documentation Tools https://wix.com - Create content/marketing websites (sometimes a basic website without coding and minimal setup is required, in these instances I have used Wix with great effect, really? Wix? yes, please read this blog post.
https://www.gitbook.com - Create good looking documentation
Tools
http://www.iconico.com/colorpic - Pixel colour picker across desktop / any app image
..and some coding utilities..
https://guidgenerator.com/ - Generate some GUIDs
I use these professionally and am personally recommending them.