You can search for music, create playlists, and play songs without having to have a tab open to Youtube. It also has a radio mode (think Spotify or Pandora) as well for finding music and keeping it playing without much attenion being paid to it. Additionally, there are keyboard shortcuts available as well to make it very easy to adjust the extension quickly.
There is a large list of features the author intends to implement such as Soundcloud, last.fm and much more.
Reddit thread from the author - http://www.reddit.com/r/Music/comments/2sypcn/seven_months_ago_i_showed_rmusic_my_software/
Subreddit for the extension - http://www.reddit.com/r/streamus
Official website - https://streamus.com/
Quick link to install - https://chrome.google.com/webstore/detail/streamus/jbnkffmindojffecdhbbmekbmkkfpmjd/