"We think this will lead to integrations that keep Spotify beautiful and simple, but layer in great musical experiences designed to be social and fun," he said.
Ek demonstrated several of the free applications created already by Spotify's partners and was joined on stage by Rolling Stone founder Jann Wenner, who described Spotify as the "ultimate jukebox."
"We'll be doing a new playlist every day," Wenner said.
In addition to playlists, the Rolling Stone app offers album reviews and other features.
An application from Britain's SongKick lets users go from listening to a song to buying tickets for a concert while the TuneWiki app simultaneously displays lyrics while a song is playing.
Other partners launching apps include Billboard, Fuse, The Guardian, Last.fm, Moodagent, Pitchfork, Soundrop and We Are Hunted.
Like Apple with its App Store, Spotify will approve all of the applications created by third-party developers before they are featured on its site.
Spotify, which was launched in Ek's native Sweden in 2008, boasts a library of 15 million tracks and is adding about 20,000 a day.
The music streaming service is available in a dozen countries in Europe and launched in the United States in July.
Spotify has more than 10 million active users and 2.5 million paying subscribers.
Spotify offers three services: a free, ad-supported streaming service, an ad-free $4.99 a month plan for computer listening, and a $9.99 a month plan allowing subscribers to download as many songs as they want to a mobile phone.