Imeem is my favourite online music community. It let you search millions of song and put it into your playlist. And of course you can share your playlist within the community and embed the player in websites.
Today this is another site called songbeat. And it claims it can let you search songs, put it in the playlist as well as embedding it in your website. Those are not surprised at all but it said you can even download it for FREE. As it is kind of new, I probably need to wait for a bit to check that site out later on.