MPEG-4 is an MPEG standard for multimedia compression that features a high degree lossy compression that maintains perceived media quality.
The most popular open source implementation of the MPEG-4 standard is the XviD codec, however due to patents on MPEG there has been some controversy over whether or not XviD is in violation.
A common format for MPEG-4 is the AAC audio encoding format. AAC is in wide use as it is Apple’s preferred format for the iPod. Open source projects exist for encoding and decorating AAC as well, notably FAAC – an AAC encoder and FAAD2 – an AAC decoder.