mpv.io
a free, open source, and cross-platform media player
SMPlayer - Free media player
SMPlayer is a free media player than can play virtually all audio and video formats. It can even play and download Youtube videos. Other interesting features: find and download subtitles, thumbnail generator, resume playback.
Fluid Player
Fluid Player, the VAST ready open source HTML5 video player
VideoLAN
VLC: Official site - Free multimedia solutions for all OS!