Audacity

In recent years Audacity has made a name for itself as the best free and open source audio recording and editing software. This reputation is not unfounded. Audacity can record live audio and computer playback. This means you can use it as a home recording studio, and convert old tapes and records. Manage multiple input and output devices simultaneously, record by timer or sound activation, utilise very high sample rates (up to 384,000 Hz depending on your hardware) and record up to 24bit depth.
You can also import and export a wide range of file formats, WAV, AIFF, AU, FLAC, OGG, MP3, AC3, M4A, WMA and more. Sound editing is made easy by Audacity as well, with the options to Cut, Copy, Past and Delete sections. You can also use Undo or Redo functions on every step you take, and an Automatic Crash Recovery system can save you hours if something goes wrong. Remove vocal tracks to create instrumental pieces. Add a wide range of effects such echoes, reverbs, revers, wahwah and stretches. Remove background noises such as hiss, hum or static. Audacity supports many third party plugins as well, giving you even more editing power. Use all these features and many more across Windows, Mac and Linux.