Cool software

Office / Documents

Dia - diagramming tool

Libre Office - Office Suite (open source)

Graphics / Audio / Video

Audacity - audio editor (open source)

Blender - 3D modeling / animation (open source)

Gimp - Extendable rasterization graphics editor (open source)

Inkscape - SVG Vector graphics editor (open source)

Programming / Text

SublimeText - Really nice, extendable text editor (commercial)

Atom - Hackable text editor by GitHub (open souce)

Self-hosted web applications

Content Management Systems

MediaWiki - The wiki used for Wikipedia (open source)

DokuWiki - Best.Wiki.Ever! Very easy to setup and maintain. (open source)

File clouds

Nextcloud - Multi-user cloud [PHP, MySQL] (open source)

ownCloud - Multi-user cloud with extensions for calendar, contacts etc. [PHP, MySQL] (open source)

cozyCloud - Single-user cloud with extensions for calendar, contacts etc. [nodejs] (open source)