List of lossless and high-res music (FLAC, ALAC, AIFF, DSD, DXD, etc.) download sites.
While the first question is answered by the wiki, the second, whether hi-res files are worth bothering with, is somewhat more complex. I need to start by saying that there isn't a definitive answer and many people disagree about it. This article is only intended to give a summary. More importantly, remember that we are here to enjoy listening to music above anything else and it isn't worth stressing over the fine details of this excessively.
First of all, normal "CD quality" music has a sample rate of 44.1 kHz (allowing enough samples to be taken to reproduce music up to 22.050 kHz -- beyond the limit of most people's hearing) and a bit rate of 16, which allows 65536 possible levels of volume for each sample. This can be written in simplified form as: 16/44.1. This allows enough range in frequency and a low enough level of noise during digital-to-analogue conversion (during playback) to cover our regular hearing range well enough.
When music is recorded and processed in a digital format, to allow editing without significant degradation in the quality, it is recorded at 24/96 or 24/192. Some sites are offering download of the "studio master" digital files at these rates, as well as 24/48, 24/88.2 and 24/176.4. These formats have a few possible benefits.
First, being 24 bit versus the 16 bit of CD quality audio, they reduce one of the issues that comes up during playback. As the levels of volume of music are infinite, reducing them to 65536 possible levels introduces some unpleasant, if low-level distortion (quantization errors). To rectify this, during playback, a very small amount of noise is added by your DAC as that is more pleasant than the distortion. 24-bit audio has 16,777,216 levels of volume, so the amount of distortion (and thus noise) is far lower during playback. Note that the noise level of 16-bit audio may even be lower than your DAC is capable of resolving (and your ears capable of hearing), however the benefit of 24-bit audio is measurable. Even so, as studio quality files are down-sampled for CD quality audio, any quality issues of the software doing the down-sampling are eliminated by getting the studio master itself.
Second, during playback, CD-quality audio can introduce distortion at the highest frequencies, close to the 22.050 kHz limit. This is due to the filter a DAC uses to cut off sounds above that frequency. However, since most instruments only produce sounds up to about 12 kHz and most people can't hear much, if at all, beyond 16 kHz, it is mostly a non-issue. Higher-resolution files from 48 kHz and up will move issues with playback into frequencies very far beyond our hearing limit. For example, 48 kHz audio has a frequency limit half that: 24 kHz. 96 kHz audio has a frequency limit of 48 kHz. Most DACs (Digital to Analogue Converters) during playback over-sample the audio (that is, convert 44.1 CD quality audio to up to 8x that frequency, 352.8 kHz) to effect the same thing, a hi-res file may, arguably, bypass any quality limitations of a DAC's over-sampling algorithm by being high-resolution in the first place. Especially people who prefer Non-Oversampling DACs (as they believe the sound is less "digital" and more enjoyable to listen with) are likely to find significant benefit when those DACs can use hi-res files to effect a non-artificial high sampling rate. The degree of benefit, however, is considerably variable and very debatable. More so now that a couple of sites are offering DSD downloads derived from recordings made for SACDs (Super Audio CDs), with some DACs capable of direct playback of these non-standard audio files.
The problem with both things above is that they deal with differences that are at the limit of or likely beyond our hearing capabilities and any audible differences may be more attributable to the behaviour of the electronics used for playback than an actual increase in resolution. However, many people feel that hi-res audio formats such as SACDs and studio master, especially those people who have spent very considerable sums on high-quality audio gear feel they are getting an audible benefit. Similarly, we may wish to get the best recordings possible and get the best out of our equipment, which leads me to the main benefit of getting music from one of these sites:
High quality recordings.
I remember, back when I had already spent quite a bit on better headphones and other gear, getting out some of my old favourite albums and being blown away being able to hear them in such clarity. Sites such as HDTracks have been working to get hold of original tapes and digital masters of many famous albums from a variety of genres and make them available in a higher quality than was previously available before. Being able to eliminate any number of compromises made when an album was produced and re-release it can make the music more enjoyable to listen to with high-quality gear. Especially given the propensity for recording studios to release albums that are highly compressed during mastering, sometimes to the point of very audible distortion, this is a very welcome alternative (See the Loudness War).
Also, a number of sites produce their own direct-to-digital or direct-from-tape masters. The recent Chesky binaural recordings are a great example of this. On the Amber Rubarth album "Sessions from the 17th Ward" not only can I hear the artists shifting in their seats but I can hear birds tweeting outside the hall on at least one track, such is the clarity. There are also many great live recordings, such as from the Cowboy Junkies available from Archive.org. One of their live albums was "made for headphones" and is fantastic to listen to. Metaxas Audio on their Reel to Reel site also has a range of direct-from-tape recordings and is some of the most amazing music I've heard. Then there is the Tallis Scholars group that has been carefully researching and reproducing Renaissance choral music, recorded in surround sound, which is amazing, even if you're not a fan of choral music. For the classical lover, Linn Records has an absolute wealth of great recordings and if you're after Asian music, Hifitrack has recordings from Chinese artists available.
So, given the 30-odd sites listed, whether you'd like some high quality music, or you're just after something different, there is plenty of good, well-recorded music available for you to explore to get the most out of your headphones.
