As you go about designing and coding your application's screens, here are a few things that you may find useful.
I've placed them in rough order of my opinion of their value/length ratio.
In rough order of how often I use them.
Font Awesome - Free, CSS-customizable, perfectly scalable icons for almost everything.
Bootswatch - Cheating. Great for quick styles while prototyping.
Google Web Fonts - You'll want this after reading Butterick's.
Stock Up - A search engine for free stock photos.
Shoelace.io - A tool to help you create your Bootstrap grid layout.
HTML5 Element List - An excellent, categorized list of HTML elements.
Bootstrap CDN - Instantaneous
<link>ing to Bootstrap or Bootswatches.
Firefox Tilt3D Extension - A tool to help you visualize what's going wrong with your layout.
CSS3 Generator - Generate CSS box shadows, text shadows, and gradients.
Subtle Patterns - A library of free, tileable, subtle background images.
Adobe Color - Generate complementary colors.