Found this because it's what Google has started using for its music search song plays. In short, an MP3 purchasing site that also sells "web versions" of songs for $0.10 each that aren't downloads, but unlock the song for your account to be listened to in-browser anytime, anywhere, on any computer. Additionally, you can download a program to scan your computer's MP3s which will then cross-reference those with the available songs in LaLa's catalogue, and those MP3s you already have will be unlocked as well, to allow you to listen to that music on the web.
Neat site, slick interface. If you want to sign up, use this referral link if you don't mind, and it'll pass me five web song credits.
If you don't want to do that, the main link is simply http://www.lala.com.