Video and Movie

Avidemux

Avidemux is a free video editor designed for simple cutting,
filtering and encoding tasks. For Windows, Mac OSX and Linux.
https://www.berlios.de/software/avidemux/
https://sourceforge.net/projects/avidemux/

DVD Flick

Convert and burn almost any video file to a playable DVD.
https://www.dvdflick.net/
Download from sourceforge.net
https://sourceforge.net/projects/dvdflick/files/

Media Player Classic - Home Cinema

Home Cinema is based on the original 'Media Player Classic' created when Gabest, the author, stopped working on the project. MPC-HC is an extremely light-weight, open source media player for Windows.
https://mpc-hc.org/
Sourceforge project page...
NB: The project has moved from Sourceforge so these pages may be out of date.
https://sourceforge.net/projects/mpc-hc/
Download:
https://mpc-hc.org/downloads/

Handbrake

Convert video from nearly any format. Open Source software available for Windows, Mac and Linux.
https://handbrake.fr/
Download:
https://handbrake.fr/downloads.php

SUPER

SUPER Simplified Universal Player Encoder and Renderer. If you need a simple, yet very efficient tool to convert (encode) or play any multimedia file, without reading manuals or spending long hours training, then SUPER is all you need.

NOTE:
1) The download link can be a little hard to find. 
2) The download of the free version contains unwanted other software. So be aware...
3) The paid version with no unwanted other software is ~US$17.00.

SUPER for Windows.
https://www.erightsoft.com/SUPER.html
Downloads:
https://www.erightsoft.com/Superdc.html

VirtualDub

VirtualDub is the fastest tool for capturing, processing and re-encoding video files from one format to another.
https://www.virtualdub.org/
Download:
https://www.virtualdub.org/download.html

VLC Media Player

VLC is a free and open source cross-platform multimedia player and framework, that plays most multimedias files as well as DVD, Audio CD, VCD, and various streaming protocols. Not particularly good looking but a geat player. Available for Windows, Mac and Linux.
https://www.videolan.org/
Download:
https://www.videolan.org/vlc/

SMPlayer

SMPlayer is a another great media player. SMPlayer is a free media player for Windows, Mac and Linux with built-in codecs that can play virtually all video and audio formats.
https://www.smplayer.info/
Download:
https://www.smplayer.info/en/downloads

ffdshow

If using Windows you may also need FFDSHOW. ffdshow is advanced DirectShow filter providing multi-format decoders that enable you to playback many audio and video formats.
https://ffdshow-tryout.sourceforge.net/

FFmpeg

A complete, cross-platform solution to record, convert and stream audio and video.
https://ffmpeg.org/
Downloads:
https://ffmpeg.org/download.html