A few months ago I started using Amazon's Cloud Drive service to store some of my music online. This was pretty much a no-brainer since I often buy MP3's from Amazon anyway, and these are now stored for you by default (and for free) on their cloud. You can also download them if you so choose. I don't have a huge amount of music there, about 2000 songs, mostly recent Amazon purchases. It includes about 5 GB of selected music I have uploaded, but I didn't really want to "commit" to Amazon's cloud service too quickly (other than the free part) since I assumed other things would be coming along, which of course is true. I do listen to the Amazon Cloud Player quite often since it includes a lot of my recent music, some of which I haven't bothered to download, especially a few 99-track classical collections bought on sale for $1.99 or so. The player has limited features but works quite well.
A few days ago I received an invitation to join Google Music Beta (I had requested this in May when it was announced). This is a pretty good deal, since Google will store up to 20,000 songs for free (at least for now - I haven't really read the fine print, but Google doesn't seem to worry much about storage as long as they have your eyeballs). The catch is that you have to upload all the songs you want them to store for you. To help you with this task, they can look at your iTunes or Windows Media Player music files to figure out what to upload (pretty much automatically, including iTunes playlists).
So it's an easy thing to do, but if you have a lot of music as I do, it's a long process. I've had it running on a good internet connection for two full overnights as well as all day today, and so far it's got about 5,800 of my estimated 20,000 songs online. I haven't paid too much attention to it except to correct occasional album cover art errors (I have a lot of MP3's ripped from CD's, most of which have cover art in iTunes, but Google sometimes gets its own and sometimes chooses wrong). The player is good though also quite basic, but this is "beta" so I assume this web-based interface will improve over time. You can start listening to music as soon as you have a few files there, which is cool. I think of it as a playable off-site backup for my music collection (though there is no provision I can see to re-download your music, so in that sense it's not a real backup).
Come fall I expect to have a third cloud full of music, once Apple introduces its full iCloud service. That will be really cool because it will scan my iTunes collection and re-create it online without my having to upload most of it (I will only have to upload anything I have that isn't in iTunes' 19 million song collection - things like vinyl albums that never made it to CD that I have ripped to MP3). I think iCloud will be the real deal because it will work directly with my iPod Touch as well as our PC's and my wife's iPhone.