File Juicer can extract artwork from MP3 files. While the ID3 standard recommend JPEG and PNG it allows for other formats as well. File Juicers raw text extraction will also find the text parts of the ID3 tags, but to get better access to the metadata ask iTunes
Wikipedia on ID3 tags
MP3 inside other formats
MP3 files can be found embedded in other file formats, PowerPoint, Word, EXE files etc, and in this case File Juicer can extract the MP3 file from those formats.
Damaged MP3 files
One special feature of MP3 files is that it can be clipped into pieces (by corrupt disk or other errors) and audio from both halfs can still be extracted and saved in individual files.
Album Wrap (.albw) files are a special variation of MP3 files, made by splicing together several MP3 files into one. File Juicer can split such files and name them if they still have the ID3 tags present. "ALBW" is likely not used as an extension, but as a part of the file name.
iTunes and mp3
You can ask iTunes
to give you information about mp3 or AAC files with AppleScript.
iTunes and Coverflow
iTunes can not only store album artwork in ID3 tags, but also in ITC files
which is made to make Coverflow fast and smooth.