WavePad
Available for both Windows and Mac, the app furnishes plenty of tools for recording and editing sound. WavePad's interface is fairly well-organized, and the Command Bar feature provides quick access to the most frequently used functions. WavePad supports a variety of popular audio formats, including MP3, OGG, WAV, and WMA, and it includes a ripping function.
WavePad offers a decent number of basic effects, such as noise reduction, normalizing, fading, amplification, echo, changing speed and pitch, and reverb, and you can preview any edits before saving. There are also text-to-speech and speech-recognition functions, impressive for a free app. All told, WavePad's many capabilities and lack of restrictions make it a fine pick for all but the most advanced, hardcore audio junkies.