Shiki - A beautiful syntax highlighter based on TextMate grammar, accurate and powerful.
Warmup Inbox - Warmup Inbox is a tool that automates the process of warming up your email inboxes, raising your sender reputation and inbox health automatically.
highlight.js - Highlight.js is a syntax highlighter written in JavaScript. It works in the browser as well as on the server.
prism.js - Prism is a lightweight, extensible syntax highlighter, built with modern web standards in mind.
Context.io - Context.IO is a REST email API that makes it easy and fast to integrate your user's email data in your application
Chroma (Syntax Highlighter) - Chroma takes source code and other structured text and converts it into syntax highlighted HTML, ANSI-coloured text, etc.