Flickr: Vagawi
When I was in high school I had to download music from Napster at 33.3kbps. I'd queue up a bunch of songs and in the morning one or two of them might have downloaded if the connection didn't crash. Today, you can find just about any song you're looking for instantaneously. Worst case scenario is you've got a crappy low bitrate YouTube upload, but you'll almost definitely find it.

But there are loads of alternatives. I wanted to figure out where kids these days are getting their tunes. So I tried out a few things and found the best songs for listening to your favorite music for free.
Good: Finding lots of music from a particular artist

Bad: Not the best for browsing, since each artist has a custom URL
Bandcamp gives indie artists and their fans a great replacement for the dying MySpace. Check out Sufjan Stevens's page, for example. Very album oriented, which is encouraging for artists who put a lot of effort into putting together full-lengths.

Bandcamp automatically generates a range of bitrate choices, offers artists robust analytics and offers all kinds of integration with Facebook, Twitter and HTML embeds.
Good: Boasts a massive streaming library

Bad: Not as feature-rich as its competitors and runs ads
While not yet available in the US, Spotify is owning in Europe. It's a downloadable app that streams music from a library of over 10 million tracks. It'll integrate with mobile devices and allow you to share stuff with friends. It is also beloved for its pretty interface.

According to this interview with Spotify's creator, the service will be available Stateside sometime in 2010.
Soundcloud
Good: Slick, quick sharing for your blog

Bad: Not designed to expose users to new music
Soundcloud is the best if you just want to share music with a friend or hundred very quickly. I've used it in the past to share with you songs that I slowed down by 800%. There are no upload size limits, so it's perfect for, say, uploading a half-hour-long Justin Bieber slow-mo remix. Uploads are fast, and embeds are very simple for blogging.
Good: Have fun with numbers and sharing with friends

Bad: Exposes all your guilty pleasures; need to get the actual music elsewhere
This music-based social network has been around for a while, but it's still a great place to share music with friends and offers lots of fascinating statistics about who is listening to what. Download the application, which "scrobbles" the music you play, uploading information to their servers and crunching the data into the sort of statistics that music geeks salivate over. It's the sort of thing that my friends and I loved to play around with in college, and it's still going strong.
Good: Easy to keep your finger on the pulse of the blogosphere

Bad: Not much good if you're looking for radio pop
The Hype Machine is also getting up there in years, but it's still the best place to find the buzziest music. It aggregates mp3 uploads from a selection of the hippest music blogs and offers the results as a set of streaming downloads. It's by far the cleverest way to predict what the next indie (or otherwise) hit is going to be.
Good: Scary smart streaming music mind reader

Bad: Kind of authoritarian about what it plays, just like a real DJ
Even music geeks like me who think they've already discovered everything can't help but enjoy firing Pandora up once in a while. It's a streaming music service that acts like a radio station that customizes in real-time according to your tastes. If you tell Pandora you like a song, it'll give you more stuff that meets certain criteria matching that song. The downside is that you have to rely on the service's algorithm. You have little control over what plays. There's also a 40 hour/month limit unless you pay a fee.
Good: Find pretty much anything

Bad: Crappy sound quality
Kind of a last resort in terms of sound quality, but it's still the best place to instantly find the rarest of tracks. And if you're lucky, you might find some live footage or an obscure video you've never seen before. I can only think of one song I haven't been able to find on YouTube ("Instant Club Hit" by the Dead Milkmen, anyone?).
Good: Lots of Stations

Bad: Not customizable
Gotta give a shout out to our buddies at AOL Music. AOL Radio just got a nice facelift and boasts over 350 radio stations. There's even a station that plays nothing but The Notorious B.I.G. 24 hours a day.
Good: More freedom than Pandora

Bad: Clunky interface
Grooveshark was recently kicked out of Apple's App Store due to the questionable legality of some of their uploads, but they're plugging along anyway, having integrated new social features. Grooveshark is a bit like Pandora in that it allows users to like or dislike songs and updates playlists accordingly on the fly. A lot of folks seem to prefer Grooveshark because it does everything Pandora does, but it allows you to save favorite songs and skip as many songs you like. Pandora imposes limits on this, presumably to behave more like traditional radio and avoid lawsuits. Grooveshark also allows you to listen to entire albums by a single artist.
Good: Giant library, slick iPhone app

Bad: $10/month
Mog offers unlimited downloads for its subscribers, but it charges a $10 monthly fee. It's received accolades for its iPhone app. It also harnesses the power of over 900 music blogs to generate recommendations and information about new tunes.