LearnGitBranching is an online tool that provides an visual and interactive way to learn Git. It is a pure clientside application written in JavaScript, only an HTML page with some JS and CSS, there’s no AJAX requests. Hence you can download the page and use it locally. It offers step-by-step demonstrations of powerful features and various levels that you can pick to challenge. See more on its GitHub repository.
Pandoc is a powerful document conversion tool, it is called swiss-army knife in this field. It understands many document formats, like markdown, ms word, pdf, html, etc. And it is free, you may install it as a command tool or try it online.
cmd dev2 apps to manage your code snippets in local machine: massCode and SnippetStore. Both tools are open source apps with different features. You can choose one according your needs.
devcheat.sh is a pretty useful tool that provides a cheat sheet for all you need like Linux commands, programming languages and DBMSes. You can use it to display usages of a Linux command, a code snip of a programming language like code for looping an array with PHP. For the moment, it covers 56 programming languages(C, awk, bash, PHP, Python, Perl, R, Ruby, JS, Go, Swift...) and 1000+ Linux commands.
cmd dev git-bash