There are many tools in this category, but few so comprehensive and efficient as Audacity. It is a free, open source software with GNU licensing to record and edit audio, working directly with the graphics of the sound waves. It can also convert files.
It supports the majority of audio formats, such as wav, mp3, Ogg, wma, ac3, flac and aiff. It also includes all kinds of basic editing tools like cutting, copying, erasing. Thanks to the amount of available filters and effects , Audacity is much more versatile than it first seems.
Besides all this, it allows you to adjust the level of noise on an audio track, modify the shape of the wave to adjust it to your needs, or add all kinds of post-process effects. Its power lays in the possibility of applying changes to any section of the audio file you want, and as well as its multi-track editing. You can even apply a filter that erases the voice in a song.
As if this wasn’t enough, you can add all types of 32 bit external VST plugins, and also reproduce MIDI files.