There's always a fear when you stumble across something a little bit special that you shouldn't share it with anyone, lest popularity ruin it. Grooveshark, for a long time, was that thing, but now you're being let in on the secret - just don't tell anyone else, okay?
The premise is simple: Grooveshark lets users upload tracks to its service, which are then available for listening to by anyone with a web browser. It sounds unbelievable, and yet somehow Grooveshark has worked out licensing deals to allow it.
The free access is blissfully uninterrupted by advertising in the audio streams. The website itself is ad-supported, but only by display advertising which is far less intrusive. VIP access removes the adverts but they're so inconspicuously integrated with the site that you'll likely never find them an issue anyway.
There's more to be had if you stump up for the $3 a month, or $30 a year VIP access, via a desktop or mobile application. The Grooveshark mobile offing is the most comprehensive around. The iPhone app is only available through Cydia currently, while Grooveshark seeks approval from Apple to put it in the App Store, but Blackberry, Palm and Android users can grab the appropriate versions of it with no hassle.
This user-provided music library is both the biggest strength and the biggest weakness of Grooveshark. A strength, because it means there's every chance you'll be able to find particularly obscure music not available on other platforms. But it's a weakness because there are generally numerous similarly titled tracks available for any given search. However, Grooveshark's filtering does a great job of weeding down search results to show only a single album's tracks - despite the multiple options.
Other nifty features include the option to share and embed playlists you've created elsewhere, or Last.fm integration (for VIPs) and more are being added all the time. And that desktop app - it's written in Adobe AIR, which means it's available on pretty much every OS going.
Considering you don't even need an account to use the free service, you should really just go try it out now.
While Spotify is lauded in public for its innovation, if that's what you're after then Grooveshark takes the crown. It's also the most adaptable of services looked at, as it's available in desktop, mobile and web browser-based incarnations.