NEW YORK (Business 2.0) - Call it Skype envy or the inevitable search for the next killer app on the Internet.

But Web powerhouses like Google (Research), Microsoft (Research), and Yahoo! (Research) are all scrambling to get in on the voice-over-Internet-protocol craze. Yahoo! bought VOIP startup DialPad in mid-June, and both Microsoft and Google are rumored to be eyeing another VOIP company, Teleo.

Today VOIP is just used to make calls over the Web. But it's shaping up to be so much more. Internet phone service could one day be as popular as e-mail and instant messaging (not to mention the threat it poses to the plain old telephone). But the really exciting thing about VOIP is its potential as a whole new platform for software applications linked to your phone calls.

The most popular VOIP application is Skype, which boasts more than 40 million registered users (a number that has caught the attention of every major Web player). The company has grown its user base simply by offering a piece of software that turns your computer into a phone. You can call PC-to-PC for free and can call regular phones around the world for about 2 cents a minute.

But Skype is not compatible with other VOIP services. Since the value of the service is its network of more than 40 million users (and the greater probability that someone you want to talk to is on Skype, rather than on another VOIP network), Skype is not likely to open itself up to rivals anytime soon.

Any Web company with a large number of members -- whether it's Google, Microsoft, Yahoo!, or a social-networking site like MySpace or LinkedIn -- cannot sit on the VOIP sidelines much longer. Phone calls, in the parlance of the late 1990s, are a sticky app. When people use Skype, they put their contact lists (their personal social networks) into the software so they can click on a name to make a call.

Think of Skype as a Web-based contact list. It knows, at any given moment, which of your friends and business associates are online. (That's what they call "presence" these days.) If you don't want to talk to them, you can send them an instant message instead.

You can also look up the profile of any Skype user in the directory. It would be pretty simple for Skype to add other features to help its members connect and interact with one another while they are talking.

That idea of making a VOIP service more than just a way to place phone calls is what's behind a new challenger to Skype, code-named the Gizmo Project.

The Gizmo Project hopes to become a private-label VOIP service for all the major Web companies that want to match what Skype offers. Built by SIPphone, one of the companies launched by Michael Robertson (he of MP3.com and Lindows fame), it just recently became available as a public beta.

I've been testing the service for the past month. It's still working out some bugs, and, as with Skype, the sound quality is not always as good as that of a regular phone, but Gizmo's pretty good for a free phone call.

I like it better than Skype because it has some cool added features: With Gizmo you can record any conversation and save it as an audio file on your computer, view a map showing a caller's location, or select the music you want people on hold to hear. Maybe I just prefer it, though, because I'm biased. SIPphone's president, Jeff Bonforte, is a former roommate of mine. (He also personally designed the user interface for Gizmo and wrote all the hold music.)

My bias aside, the reason the quality of the calls on the two services is so similar is that Skype and Gizmo both license the same audio software from a Swedish company called Global IP Sound.

GIPS's patented software includes a codec, echo cancellation, jitter protection, and a way to connect directly to a computer's hardware -- actually bypassing the operating system -- to improve reliability. Startups like Skype and SIPphone can license the software much cheaper than the established players, which so far have not been willing to pay up. Why someone doesn't just buy GIPS is beyond me.

In the meantime, Bonforte hopes to sign up some Web sites to use Gizmo as their own VOIP software and offer it to their existing networks of users. In other words, he thinks that VOIP will just become a feature of larger Web sites, and he wants to provide that feature.

He is also opening up Gizmo to software developers so that anyone can add to it. Already, as part of the beta software, you can type in "info" and get connected to a TellMe server, which will give you voice-prompted directions, stock quotes, or weather updates, or even let you play blackjack with a computer-generated Sean Connery.

The number of voice applications that could ride on top of Gizmo, Skype, or any other VOIP service is limited only by programmers' imaginations.

A VOIP service could be created, for instance, so that podcasters could read their podcasts into a Web server and make them available to anyone who calls their VOIP phone number. After listening, people could add comments simply by speaking into the phone, and those messages could then be appended to the end of the podcasts, just like written comments on blogs today.

"One developer in San Jose," Bonforte notes, "wrote an application where you call in and tell it the intersection you are at and it will tell you what time the next bus will be there, based on GPS data." You can also build voice into existing applications like instant messaging or online games.

Or someone could write a program to set up alerts. When I'm expecting a critical e-mail from an executive at a big customer, say, I could have my VOIP service call me on my mobile phone and read the message to me when it arrives. Then, with another voice command, I could set up a conference call with my 10 salespeople, record it, and send it to everyone as an MP3 file.

"In the long run," Bonforte predicts, "voice is just another method of accessing and sending data. Someday we will wonder, Why did we ever think Vonage is such a big deal? That is just $10 a month off my phone bill. VOIP is really about applications, and their integration into our lives." He's right. Voice is data, and data is a big deal.

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