Here is an article by Matt Rosoff at Digital Noise:
Launchcast was owned by Yahoo and was the original online music service, but I don’t get the sense that its as popular as some of the others. CBS already owns Last FFM and plans on changing Launchcast to fit a different niche. Last FM and Launchast combined will create quite a competition to the other offerings.
Edit: I want to add a couple of links and comments.
The first comparison I came across was this one by Steve Krause. Its a good analysis of the differences between the two biggest players that have free services.
Here is a review by Gary Savelson that gives a quick synopsis of many different services.
This one that is very detailed.
This is interesting… a couple of sites that allow you to use your Last FM account to discover videos on Youtube.
The comment I wanted to make is that there is some nice software that helps connect between these services. There is one that sends what you listen to on Pandora to your Last FM account and one that does the same sending your Rhapsody music to Last FM.
Last FM seems to be the most popular and sounds like its probably more useful for most people. It has a simpler model and the community aspect creates a strong loyalty base. I’ve found it easy to use and it gives me solid recommendations for similar music. I’ll explore Pandora some and I might even choose to pay for Rhapsody, but whatever the case I’ll probably still visit Last FM.
Its hard to tell which services might survive in the long run as the music industry is always looking to increase their profits which might force under some of these free services. I don’t know about Pandora because it was having some trouble recently, but I’m willing to bet Last FM lasts as it has a large company backing it. Pandora could easily lose out as soon as other companies start trying the same thing they do.
I’m not sure what is going on with Yahoo. There changing everything around and I don’t know what they’ll be offering in the future. They shifted some of their services off onto Rhapsody. I have no idea how Yahoo compares, but it doesn’t seem to be in the same league with Pandora and Last FM.