With a clever pun in its name and some awesome functionality, Audacity is the next Open-Source utility I'm featuring. Somehow I neglected to mention in it my initial list, probably because I found it before I found the rest of my favorites. But it is not any less great. Audacity is an Audio Editor and Recorder, and of course it's free. With it, you can record pretty much anything - Wave out mix (that's whatever your speakers are playing, recorded in an mp3, wav, whatever on your computer), Microphone, Line in (basically a high-quality, stereo microphone jack on most desktops), CD audio. You can also open many files, of course.
Once you have your audio, you can apply any one of a long list of filters to make it sound like you want - fade in, fade out, laser wah, and noise removal, for example. One of my favorites is its ability to easily change pitch or speed independently without adversely affecting the other. Normally, when you increase speed, the sound gets higher and you get the chipmunk effect, or the sound gets lower if you decrease it. In Audacity, you can change the tempo (using any number of units, from RPMs for records to beats per minute for metronome measurements) without affecting pitch, and you can change pitch using not only percentages, but note names - if you have a piece in C, change pitch from C to F, and you've got a nicely transposed piece of music, that easy.
Its slicing and dicing tools are also helpful - I've created my own Switchfoot sound scheme for Windows by slicing up my Nothing Is Sound CD. I've also used it to transpose or slow down the audio recordings I have for my piano practicing so I can play along (useful for, say, Phantom of the Opera). So if you're an audiophile, or just like playing with sound and music, check it out. As always, you can't beat the bang for the buck.
And if Audacity happens to crash, don't fear. You can recover it with the also free Audacity Recovery Utility to get your sound back. How cool is that?