VLC is a free and open source cross-platform multimedia player and framework that plays most multimedia files as well as DVD, Audio CD, VCD, and various streaming protocols.
Simple, fast and powerful media player.
Plays everything: Files, Discs, Webcams, Devices and Streams.
Plays most codecs with no codec packs needed: MPEG-2, DivX, H.264,
MKV, WebM, WMV, MP3...
Runs on all platforms: Windows, Linux, Mac OS X, Unix...
Completely Free, 0 spyware, 0 ads and no user tracking.
Can do media conversion and streaming.
From the VLC media player home page.
I was using it the other day to play back 3 720p videos simultaneously on a 1 1/2 year old HP notebook. Performance was flawless.
Very advanced customizations/settings.
Separate window that includes playlist, file browser, external devices and local network access, internet features (radios, postacasts, Freebox TV channels for registered users)
(Using skins reduces the easy access to some of these features.)
VLC media player supports many audio and video compression methods and file formats, including DVD-video, video CD and streaming protocols. It is able to stream over computer network and to transcode multimedia files. (from Wikipedia)
It can use extensions, plugins and skins - to be downloaded from addons.videolan.org.
Among these, some that can find movie subtitles. (More answers on this: here and here.)