FAAC is the oldest free and open-source AAC encoder. It is available for practically all Linux distributions. Present version (June 2012) is 1.28.
What is FAAC?
FAAC stands for Freeware Advanced Audio Coder. FAAC is a software to compress audio in the AAC format.
Portable Reasonably fast LC, Main, LTP support DRM support through DreaM
Please note that although faac is available in source code format, its license is NOT the GPL. Quoting: "FAAC is based on the original ISO MPEG reference code. The changes to this code are licensed under the LGPL license. The original license is not compatible with the LGPL, please be aware of this when using FAAC. The original license text can be found in the README file included in the download package."
The default quality setting for faac, q=100, generates files at an average bitrate of approx. 128kbps. This quality level is good enough for casual, non-critical listening, but note that other encoders for AAC and other compressed formats may provide better quality files at similar bitrates.
For better quality encoding, I suggest q=150, resulting in average bitrates around 175kbps. Based on my own (subjective) tests, at this quality level faac provides high quality artifact free music reproduction and is comparable in quality to proprietary AAC encoders at similar bitrates.
Note that faac will by default wrap AAC data in an MP4 container for output files with the extensions .mp4 and .m4a.
A companion AAC decoder, faad2, is available.