The W3Schools: collection of free HTML, CSS, JavaScript, DHTML, XML, XHTML, WAP, ASP and SQL tutorials with lots of working examples and source code
Font Awesome: font and icon toolkit based on CSS and Less
Sublime Text: source code editor which natively supports many programming languages and markup languages
The Linux User Group (LUG) Frankfurt: transfer of knowledge in the field of free and open source software (especially Linux) as well as support for the installation and operation of such for the general public
Arch Linux Wiki: documentation and guides for Arch Linux
Wiki of ubuntuusers.de: comprehensive collection of instructions and troubleshooting