Audacity is a free, multi-track audio recorder and editor that runs on Windows, Mac OS X, GNU/Linux and other operating systems. You can use Audacity to record live audio through your computer microphone, an external mic, mixer or other line in input. For example, you can also use it to convert your records or cassette tapes into digital recordings. Depending on your sound card, Audacity can also capture streaming audio. It will let you edit WAV, AIFF, MP2, MP3 and various other types of audio files. You can cut, copy, splice or mix sounds together and dub over existing tracks to create multi-track recordings. It will let you remove static, hiss, hum or other constant background noises. You can even change the speed or pitch of a recording and there are numerous built-in effects (echo, paulstretch (extreme stretch), phaser, reverb, reverse, wahwah and others). And lastly, you can export your recordings in many different file formats, including multiple files at once.
For basic features, it is fairly easy-to-use, especially if you have used other audio recording software. Some of the more advance features are a bit hidden, but there are online manuals, FAQs and other resources to help.
Download Audacity from the SourceForge.net site.