The FAAC project includes the AAC encoder FAAC and decoder FAAD2. It supports several MPEG-4 object types (LC, Main, LTP, HE AAC, PS) and file formats (ADTS AAC, raw AAC, MP4), multichannel and gapless en/decoding as well as MP4 metadata tags. The codecs are compatible with standard-compliant audio applications using one or more of these profiles.
MP3do is an "all in one" bash script. It is useful for decoding and mastering audio files. It can decode a list of MP3 or Ogg Vorbis files to WAV or CD-R, auto-convert poor quality MP3s to 44100Hz stereo, check whether all files fit on a CD, normalize all WAV files (adjust the volume to a standard volume level), and burn a CD.
ripx is a script designed to make ripping CDs as easy as possible. It features CDDB lookup, ID3 tag editing prior to ripping, playlist file generation, album directory creation, and generally a nicer interface than many similar tools. By default, ripx uses cdparanoia to extract audio from CDs and lame to encode the audio to MP3 format at the quality of your choice. If you wish to use alternative applications for extraction or encoding, modifying ripx to handle these should be fairly painless.