Libavcodec

Libavcodec is a very well thought library for decoding/encoding video. It is part of ffmpeg and is used in mplayer. Using libavcodec is very easy from either C or C++. I used to use other libraries, but libavcodec has some advantages over all of them: 1. It is alive and very active in the development comunity. 2. It is easier to use because of its simplistic API. 3. It supports lots of codecs, including MPEG4 and H261. 4. It runs in multiple platforms. 5. It is LGPL, hence you can feel free to use it in your own projects or you can contribute to it in case you find something out of place.