Free Software

Audacity - Audio Editing
Hydrogen - Drum Machine
Wireshark - Packet Sniffing
FireFox - Web Browser
Python - Programming IDE
Zint - Barcode Software
GnuCash - Accounting software
Notepad++ - Notepad for coders
HexChat - Internet Relay Chat software
FileZilla - FTP Client
VirtualBox - Virtual Machine
GIMP - Image proccessing
Inkscape - Vector Image processing
WinMerge - 

...
virtually everything is a free software
and so many others...