A few services have been popping up lately that let you stream music from any computer or device (the so called "cloud"). Amazon just released theirs, uncreatively named Cloud Player.
I'm pretty excited about this one because it's the first I've seen to actually offer sane, reasonable pricing. First, you get 5GB free which is like an ipod nano (I have one of these and although I have to rotate music on it frequently it's a decent amount of space). There is a promo deal right now where if you buy any music on Amazon MP3 you automatically get upgraded to 20GB of free space (free trial for a year). Exceeding that, pricing is pretty good. My entire digital music collection is a modest 180GB and this would cost $17 / month. Not too bad for access to all that music on my Android phone and and any web browser (there are some hacks for ipads, it seems). I'll probably just stick with the free quota and keep albums I listen to in rotation though.
The coolest feature is that any MP3 album you buy directly from Amazon MP3 (average $9) does not count toward your storage quota -- it's basically available in the cloud for life. That's pretty sweet. I tend to buy from Amazon MP3 anyway since their downloader works really well and the files are never DRM protected. Amazon, as far as gigantic enterprises go, is pretty sensitive about not locking you into their services. So, if someone makes a cheaper cloud player in the future you could just copy all your purchased files from Amazon to the new one since they are just regular mp3 files.
In typical Amazon fashion, the experience of uploading music from my Mac was really smooth. You enter the cloud player website, click upload music and you download a program that scans all your music and lets you upload by artist, song, or iTunes playlist. I don't see a way to upload by album which is unfortunate. As a workaround you could create a "to the cloud" playlist in iTunes and keep it up to date with your cloud files. Purchasing an mp3 album from Amazon MP3 was even snappier -- in 10 seconds the songs showed up in the cloud player and in a minute they were downloaded to my local iTunes library.
Also in typical Amazon fashion I bet they will release an API soon. That might allow for some really interesting playback interfaces to emerge.
Both Google and Apple are rumored to be working on a similar player but I'm skeptical. Apple will probably find a way to make you pay obscene amounts of money and lock you into their platform ... but their interface could be really nice. I have no idea what Google might offer, we'll see.