First, we have the excellent http://www.audacityteam.org. This is a longstanding project, and has by far the most functionality of any cross-platform (Windows, Linux, Mac, others) project of which I’m aware. It can handle as many tracks as CPU and RAM will permit, and has an excellent array of effects, including the ability to remove tones from a complex whole using its Noise Reduction facility.
Additionally, we have a newcomer, http://www.ocenaudio.com.br. This is a wonderful piece of work, and is much easier when trying to do a simple edit on a single audio file. Its UI is a work of art, both visually and in function.