USEFUL SOFTWARE | Free, Open Source, Commercial

[Mudbill]

Alright, I'll throw in a few.

PC:

VLC Media Player - Quite honestly the best video player out there | Free/Open Source
WinRAR or WinZip - Archivers to use with common file types like .rar | Free/Open Source + paid versions.

Mac:

VLC Media Player - Quite honestly the best video player out there | Free/Open Source
HyperDock - Makes for incredible flow with multitasking features | Free trial/paid licence.
Little Snitch - Very useful for monitoring network usage and blocking connections | Free trial/paid licence.
The Unarchiver - Archiver tool for formats such as .rar | Free/Open Source
Also Audacity, Blender and GIMP are available here.