Spotify

Spotify has millions of active users who don't pay a cent for the service -- they just opt to hear advertising on the streaming music service instead of paying a monthly fee.

The one catch is that free listening is only available on computers. However, this could soon change.

Spotify is reportedly in the midst of planning free, ad-supported, mobile music streaming, according to The Wall Street Journal. This means users will be able to listen to music on the go via their smartphones and tablets.

According to the Journal, the purported free mobile service will let users play a few songs on demand, but will mostly act as a radio station, much like Pandora and Rdio. For their part, Pandora has long offered free, ad-supported, mobile music streaming, and Rdio introduced a similar feature in October.

Sources familiar with the matter told the Journal that Spotify was said to have completed licensing deals with the three major music labels it works with -- Sony, Universal, and Warner. Reportedly, Spotify has been working on these deals for nearly a year.

In March, Spotify said it had surpassed 6 million subscribers, a gain of 1 million since December, making it the fastest-growing digital music company ever and second in reach to Pandora. But both Pandora and Spotify are burdened by the high cost of music licensing. In Spotify's case, the company pays an estimated 70 percent of its sales to rights holders through direct deals with music labels and publishers.

In addition to its more than 6 million paying subscribers, Spotify has more than 24 million active users who listen to its library of more than 20 million songs. The service is available in 32 different countries worldwide.