Who said radio was dead? Palo Alto-based TuneIn lets listeners stream more than 70,000 radio stations from around the world as well as more than two million on-demand programs such as podcasts and TEDTalks. TuneIn just announced it has 40 million active listeners, up from a reported 30 million just last year, and raised $16 million from investors including Google Ventures and Sequoia Capital.

Listeners can search for a song, artist, or show to find a station to listen to from anywhere in the world for free. The app lets listeners browse by location or genre. Listeners can bookmark songs and radio stations. The app also has voice search for when you're driving, and a recommendation engine that lets you "shake for related." With a 99-cent subscription, TuneIn users can also pause, rewind, and record unlimited radio. The app is available on all major smartphones and tablets, smart TVs, on the web, and even in some car dashboards.

The company has had great success partnering with radio broadcasters including ESPN Radio, CBS Radio, and Fox News Radio. A search for stations around New York pulled up a list of more than 100 stations including the local NPR affiliate, Hot 97, Rutgers Radio, and several police and fire departments. "Wow wow wow great app love it to bits," reads the most recent review in Play, the Android app store.

TuneIn was founded in 2002 as RadioTime, an online directory of radio stations. Ben Alexander, an independent iPhone app developer, used the RadioTime directory to create the first version of TuneIn. The app sold for $1.99 and became a best-seller. In 2010, RadioTime acquired TuneIn and pivoted to building streaming radio apps.

Broadcasters seem eager to partner with TuneIn. Why should their audience be limited by something as outdated as a radio signal?

The company has 70 employees and is expected to use the money for a massive marketing effort, as it's still relatively unknown compared to blockbuster streaming music apps Pandora and Spotify. By streaming existing programs, it doesn't face the legal hurdles or music royalty issues other services have had. Broadcasters seem eager to partner with TuneIn; why should their audience be limited by something as outdated as a radio signal?

TuneIn faces competition from a similar app, IHeartRadio, built by radio behemoth Clear Channel. Clear Channel owns 850 American radio stations and 140 more in Australia and New Zealand. Clear Channel says 98 percent of its listeners are still tuning in via radios rather than the internet. Meanwhile, some stations across the country are seeing their online audiences increase to more than 20 percent of total listeners.

For a while, it seemed that satellite radio and music streaming services threatened to make radio obsolete. But as TuneIn and apps like it get bundled into phones and cars, an old medium is getting a new life.

"Over the course of the past 15 years, people have prematurely predicted the doom of radio," TuneIn CEO John Donham told The Verge. "So yeah, I think we are bringing radio back and it's becoming cool again, but I also think it never went away. Tape players, CD players, satellite — none of these replaced radio."