VLC (initially VideoLAN Client) is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, ...) as well as DVDs, VCDs, and various streaming protocols. It can also be used as a server to stream in unicast or multicast in IPv4 or IPv6 on a high-bandwidth network.
It is a free cross-platform media player
It supports a large number of multimedia formats, without the need for additional codecs

It can also be used as a streaming server, with extended features (video on demand, on the fly transcoding, ...)

Input formats
MPEG (ES,PS,TS,PVA,MP3)
ID3 tags
AVI
ASF / WMV / WMA
MP4 / MOV / 3GP
OGG / OGM / Annodex
Matroska (MKV)
WAV (incuding DTS)
Raw Audio: DTS, AAC, AC3/A52
Raw DV
FLAC
FLV (Flash)

Input media
UDP/RTP Unicast
UDP/RTP Multicast
HTTP / FTP
MMS
File
DVD 1
VCD
SVCD 2
Audio CD (without DTS)
MPEG encoder 3
Video acquisition

Video
Decoders
MPEG-1/2
DIVX (1/2/3)
MPEG-4, DivX 5, XviD, 3ivX D4
H.264
Sorenson 1/3 (Quicktime)
DV
Cinepak
Theora (alpha 3)
H.263 / H.263i
MJPEG (A/B)
WMV 1/2
WMV 3 / WMV-9 / VC-1 4, 5
Indeo Video v3 (IV32)

Subtitles
DVD 6
SVCD / CVD
DVB
OGM
Matroska
Text files (MicroDVD, SubRIP, SubViewer, SSA1-5, SAMI, VPlayer)
Vobsub

Filters
Deinterlace
Cropping
Image wall
Image adjust
Rotate/Mirror
Logo overlay
Magnification
Image distortion
Bluescreen
RSS/Atom feeds

Outputs
Native DirectX, GDI
SDL
ASCII Art

Audio
Decoders
MPEG Layer 1/2
MP3
AC3 - A/52
DTS
LPCM
AAC
Vorbis
WMA 1/2
WMA 3 7
ADPCM
DV Audio
FLAC
QDM2/QDMC (QuickTime)
MACE
Real Audio 8
Speex

Filters
Visualization effects
Equalizer

Outputs
Native DirectX WaveOut
S/PDIF DirectX WaveOut
Multi-channel DirectX WaveOut
SDL

Streaming
Interfaces and control
Default WxWidgets
Qt 4
Skins
Web
Telnet
Command line

Miscellaneous
SAP/SDP announces
Bonjour protocol
Mozilla/Firefox plugin
ActiveX plugin
SVCD Menus
Localization
CD-Text 10
CDDB CD info 11
IPv6
IGMPv3 Win XP
CPU acceleration 13
Additional skins are available at the developer's website.

What's New
fullscreen behaviour on Windows with multiple screens,
workaround bug with libxml2 >=2.7.3,
video performance on Intel-based Macs,
various decoders updates on Windows.