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 developers website.
Whats New in version 0.8.6h:
Updated GnuTLS and libgcrypt on Windows and Mac OS X (CVE-2008-1948, CVE-2008-1949, CVE-2008-1950)
Updated libxml2 on Windows and Mac OS X (CVE-2007-6284)
Updated libebml and libmatroska on Mac OS X. Reliability improvements.

The license of this software is Freeware, you can free download and free use this media player software.