As mentioned before, Last, Blip, Imeem and Thesixtyone are the top 4 websites from where people listen music online and share them to twitter.
According to Mashable’s poll, besides those 4 top websites, some people like to listen and share their music to twitter from another 3 websites:

You can not only share the music to Twitter, but you can also share them to FaceBook and StumbleUpon, and what is more, you can embed the music to your own websites.

You can share the songs or the blogs you like to Twitter and last.fm directly.

Via Shoutcast, music can be shared to Twitter, Facebook, Delicious and so on.
Now, you have at least 7 choices to listen and share online music to Twitter for free, please let me know which one is your favorite.
Related free web apps:

