foobar2000 (
Wikipedia,
official site)
This is a combination player/transcoder/ripper/library/CD writer.
It has no eye candy, so is usually popularly ignored, but it's favored by technical audio enthusiasts. The project is the spiritual successor to Winamp and started thriving around the time Winamp stopped focusing on quality and started focusing on eye candy, marketing with strange version numbers, and needlessly forking into different versions (losing many of its features in the process).
It is extremely feature-filled, well-organized, and has a huge focus on quality (I have virtually never encountered bugs in it). Strongly recommended.
Definitely recommended preferentially over Audacity, iTunes, and SUPER (have used all of these), if you want to use it for audio conversion (obviously, some of those programs are specialized in other areas--SUPER for video, for example--though there are
much better video conversion programs than SUPER...).