This is another Swiss-Army Knife converter, letting you move back and forth between AVI, MPEG-1, MPEG-2, DivX, and SVCD formats.
Version 6.7 is shipping as of June, 2007.

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VLC is an Open Source multimedia player that supports your usual mix of audio and video formats. Support is currently included for MPEG-1, -2, -4, h.264, mp3, ogg and more. The site includes precompiled binaries for a really nice range of platforms, including Windows, Mac OS X, varios Linux flavors, and even BeOS!
Version 0.8.6 of […]

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The FFmpeg project consists of two main parts: FFmpeg, which encodes and decodes the multimedia streams, and FFserver, which provides streams via HTTP for various multimedia clients. FFMpeg is completely portable since it does not rely on proprietary DLLs. The library libavcodec, which contains all the ffmpeg codecs, can be reused in any program licensed […]

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A library the provides the functions you need for editing MPEG streams. MPEG streams were really not created with editing in mind, so a library like this needs to take into account a lot of quirky things.
Version 1.7 is shipping as of June, 2007.

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The Quasar DV codec (libdv) is a software codec for DV video, the encoding format used by most digital camcorders, typically those that support the IEEE 1394 (a.k.a. FireWire or i.Link) interface. Libdv was developed according to the official standards for DV video: IEC 61834 and SMPTE 314M.
Version 1.0 shipped in March, 2005.

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GSpot identifies presence or absence of “B-VOP”, “Qpel”, and “GMC”, thus allowing the user to determine if a DivX5 file is compatible with KISS and other popular hardware DivX players. Provides information about MPEG1 or MPEG2 video and/or MPEG1 or MPEG2 audio either as “elementary” streams or multiplexed into an MPEG1 “system stream” or an […]

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Xine is a free gpl-licensed video player for unix-like systems. It plays mpeg-2 and mpeg-1 system (audio + video multiplexed) streams,
mpeg elementary streams (audio/video only, e.g. mp3 files) and
AVI files (using win32 codec dlls). Xine plays the video and audio data of the stream and synchronizes the playback of both. Depending on the properties of […]

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This SDK decodes and encodes H.264 AVC (Advanced Video Coding) frames, making it easy to add support for this exciting new format to your applications. The MainConcept H.264 Codec will offer ultra-high quality encoding to and decoding from H.264 material at exceptional speeds in all profiles and levels supported by the standard.

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FFDShow is DirectShow and VFW codec for decoding/encoding many video and audio formats, including DivX and XviD movies using libavcodec, xvid and other opensourced libraries with a rich set of postprocessing filters.

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This is the current home for the DivX encoder and decoder. DivX ships in three different versions which offer varying levels of features for varying prices (including free.) Shipping DivX Video 6.6 as of May, 2007.

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The MPEG4IP project provides an MPEG and IETF standards-based system for encoding, streaming, and playing MPEG-4 encoded audio and video.
Version 1.5 is shipping as of November, 2006.

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This project is developing an open source MPEG-4 codec. The code is currently ported to Solaris, Win32, and Linux.
Version 1.1.3 is shipping in June, 2007.

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Here Fraunhofer IIS institute provides description of it`s AAC related technologies.

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