Still the best sounding headphones under $20

So your iPod earbuds died, and you're looking for replacements that are cheap but sound good. Sound like you? I was in this position awhile ago and decided on the Koss KSC75 as a replacement, based on glowing reviews on several forums. So why are these so great?

Sound
It's somewhat difficult to describe sound quality in general, but bear with me while I try. There are different levels of "faithfulness" to the original sound; in other words, some headphones are very accurate but don't sound good because of either the environment or because of people's expectations, while other headphones are biased in some of their frequency ranges which sometimes tricks people into thinking it sounds better, such as headphones that are bass-heavy.

These headphones are pretty well balanced. They're not completely accurate, as it sounds to me that the bass is boosted slightly, and they're a bit weak on the high frequencies, but for most listening, these traits actually make the music sound better. Let me give you my short opinion on some other headphones/earbuds, just to give you some reference points:
Stock Apple iPod earbuds - the iPod earbuds are not bad, but they lack bass, sound somewhat "tinny" at higher frequencies, and have a muffled midrange.
Koss "Plug" - great sound isolation, but very bass heavy and very very poor high frequency range; sounds even more muffled than the iPod earbuds.
Koss A130 - very accurate sound reproduction, excellent across all frequencies but weak bass for my taste.

Compared to these three headphones, the KSC75 are closest to the A130 in sound quality. In comparison, the KSC75 has a slightly tinnier high range, but the A130 are much larger, closed headphones. So the fact that the KSC75 are this cheap, portable, and have excellent sound quality is amazing.

Fit
These were kind of strange to put on at first since they clip around the back of the ear, unlike many other headphones. However, once you've put them on a few times, you can do it one handed with gloves on! Once on, they are light and don't hurt my ears, yet are firmly attached. Note that the sound quality is slightly improved if you have them tight to your ear; I had to bend the clips a little to achieve this. Also note that if you tug really hard on the cable, the clips will detach; this is probably better than having your ear being tugged off your head, and the clips just snap back on.

Usage (or, everything that didn't fit elsewhere)
The cord length is appropriate for a portable audio device (it's long enough for me to have my iPod in my pants pocket without too much slack left over), and I like the Y cable. Some of the KSC75 come with a volume control; my first set had it, and I never found it very useful (I just had it turned all the way up most of the time). It also tends to wear out and make the sound staticy, so I'd recommend you find one without the volume control.

My first pair died after about a year. One of the cables (the left one) had gotten strained and was making intermittent connection. This is somewhat due getting it caught on a lot of things, but it should be expected for any portable set of headphones. What differentiates Koss from nearly every other headphone manufacturer is their lifetime warranty; I just mailed them back with a note and $6 and they sent me a new pair in a week.

As for looks, they look strange due to the ear clip design, but hey, it's the sound that matters right?

One thing these headphones are NOT good for is sound isolation. If you're looking for headphones to use on an airplane or train, these won't work that well. You need to move to in-ear earbuds, like the Koss Plug or Creative EP-630.

Summary
Cheap, great sound quality, lifetime warranty, what else do you want?