Spotify is popular because it is reliable and fast. It essentially acts as a giant external hard drive for its users, offering full songs and albums as if the listener owned the album outright. It also offers a recommendation and search feature.
Spotify's unlimited service is free for the first six months of use. After that time, a user can continue its service for free on a limited basis, or subscribe to unlimited use for $5 per month.
Pandora
Pandora was the most well-known and widely used Internet radio station in the US until Spotify came around. Its main feature is a "recommendation engine" which essentially learns your particular music habits - your favorite bands, the style of music you like to listen to - and offers playlists based on those preferences. Pandora's popularity is limited to the fact that it is only available in the United States.
Grooveshark
Grooveshark allows the user to design and create their own music playlist with its playlist creator. It offers hundreds of thousands of songs from its catalog, giving its user the ability to create a personalized playlist that can satisfy even the most picky listeners. Using the service is free, but users will have to contend with an advertising sidebar on the screen. For $3 per month, listeners can enjoy Grooveshark without the ads.
Acoustic Alternative
This radio station, as one can ascertain from its name, specializes in acoustic music. However, plenty of other music genres are offered to its listeners as well, such as alternative, ska and punk. Though not as popular as, say, Spotify or Pandora, Acoustic Alternative enjoys high reviews from its users.
Maestro.fm
Maestro links music with social networking. Through the website, users can share their playlists with other listeners and engage in discussions with fellow users. This allows users to learn about up-and-coming artists or new styles of music that they may not have become exposed to otherwise.