Apple just launched a new section of the iTunes Store called "Mastered for iTunes", which includes tracks specifically mastered to sound good at low bitrates -- no matter what kind of audio equipment you're using.
We've talked about bitrate before, and it's pretty simple: the lower the bitrate, the more compressed and low-quality your music becomes. However, since it saves space, most online music stores, like iTunes, sell music in low-bitrate formats so they're quick to download and fit on your mobile devices. The problem is, many tracks these days are mastered for low-end audio equipment, like your iPod earbuds, with things like bass boost added to make them sound less "thin". As a result, they sound bad when you try to play them on good speakers or headphones.
Mastered for iTunes tracks are AAC, as usual, but are mastered with a specific set of guidelines in mind, designed to sound good on any type of audio equipment. Essentially, they're mastering the tracks so that when you downconvert them to AAC, you lose the least amount of data possible. They aren't lossless, but they're designed to be a better compromise between space efficiency and audio quality.
Ars Technica has a great writeup on the ins and outs of the process, and Tested does a good job of summing it up in a more concise, easy-to-understand way, for those of you interested in the process.
For the rest of you, you can check out these new Mastered for iTunes tracks in the iTunes Store right now. Right now there are about 100 albums available, from popular artists like U2, Nirvana, Metallica, Coldplay, and more, with others to be added soon. They look like they're the same price as non-Mastered for iTunes albums too, which is great. Head to the iTunes store to check it out -- you should see it right on the front page.