Online Music Streaming Services

In this field I see that there are really 4 competitors,

1. Pandora
2. Spotify
3. Grooveshark
4. Last.fm 

There certiantly are more than that but the other notable ones are only by pay services and, well, I've never been one to pay for music, anyway anyhow. My personal favorite out of these is the Pandora service, I've been using it for quite a few years and has been quite good and the music never gets old for me. There are always new songs on the stations etc.

Pandora

My favorite, has a nice site, even though it still uses Adobe Flash (the horror!) for the music playing. Also has a free to use Android and iOS app which makes streaming away from the computer convient and easy. As far as desktop apps go Pandora has 'em beat when it comes to linux desktop clients. For linux there is Pithos, which supports most if not all of the main features of the website. In addition with pithos there are no ads! Like I usually say if you really do support what the company is doing than you should show them by buying their services.

Spotify

 Spotify, arrived in the US to much fanfare, but I like many others was disappointed by the requirement of having to have a facebook account to register with spotify. In addition to this facebook has now integrated what you play and what other people play so that everyone can see what you have been listening to. This is not so much a matter of that I don't want other people to know what kind of music I listen to but another unecessary and trival thing that will fill my facebook feed. The desktop client is as good as any, it like many other things these days is trying to become more "social". For mobile Spotify is on par with most of the other services; they offer the "radio" feature for non-premium users.

Grooveshark

With their recently reinstated mobile app grooveshark looks like most of the other players listed here. Like others here the website version of this service uses Adobe's Flash player. But there is a html5 beta version that you can try. It's basically just the mobile version of the site. There is a desktop version, I have never used I personally so I can't talk.

Last.fm

  Last.fm definetly focuses more on the radio and the exploring new artist than just the listening to what you want. Last.fm is also about the live shows; it will try and sell you tickets to a concert of a band that you are listening to if you are in the area. This can be a good or a bad thing, it depends on who you are.