Some awesome open-source softwares:

ClamWin - Free & open source antivirus      

Joomla - Free & open source content management system

VLC media player - Free & open source media player

Audacity - Free & open source digital audio editor

Drupal - Free & open source content management framework

LibreOffice - Free & open source office suite

Mozilla Firefox - Free & open source web browser

WordPress - Free & open source content management system

MySQL - Free & open source relational database management system

Linux - Free and open source operating system

OwnCloud - Free & open source self-hosted file sync and share app

Filezilla - Free & open source file transfer

Notepad ++ - Free & open source code editor

KeePass - Free and open source password manager

Virtual Box - Free and open source desktop virtualization software

Wireshark - Free and open source packet analysis tool

7zip - Free File Manager

ClipGrab - Free downloader

Code:Blocks - Free & open source, cross-platform IDE