There are more than 1,000 business incubators in the U.S. But there’s only one pro bono incubator. No, not a professional startup camp run by the U2 frontman. Which is probably a good thing, since Bono was named the 2010 worst investor in America. (OK, that was before he hit home runs with Facebook and Dropbox.)

Pro bono as in free. EvoNexus (which does sound kind of like a U2 album), is a project of San Diego tech industry association CommNexus, which launched the incubator in 2009 to stimulate the growth of new high-tech companies in the area.

Like most other high-tech incubators, EvoNexus provides entrepreneurs with office space, mentoring and VC networking. But unlike other incubators, those entrepreneurs pay no fees and give up no equity.

“We believe we’re the nation’s only pro-bono high-tech incubator, no strings attached,” says EvoNexus operations director Bailey Cunning. “Our sole goal is to grow the tech sector in San Diego. And we want to do it without making companies give something up.”

It’s an attractive proposition. And it’s drawing a lot of interest from entrepreneurs. EvoNexus has had more than 300 applicants since it started - a third of them in the past eight months. About one in 18 is accepted, and there are currently 23 entrepreneurs in residence at the program’s two office spaces in downtown and suburban San Diego. Startups stay for an average of a year and a half, a lot longer than the three-month tenure at a typical for-profit incubator.

EvoNexus participants get facilities, utilities, broadband and all the business services they need. They get help with their pitches and free lunches with VCs. They get to choose their own mentors based on industry fit. And they get San Diego beaches, sunshine and the Gaslamp Quarter to unwind after a hard week of development.

“Applicants are usually pretty shocked that we offer all this pro bono,” Cunning says. “Usually they want to apply immediately.”

Twenty-nine startups have been incubated to date, and six have “graduated,” which means they’ve gotten a VC or strategic investment and left the nest. Graduates include EcoATM (recycling kiosks for consumer electronics), IO Semiconductor (fabless semiconductors), TretraVue (3D measurement and imaging), Pixon Imaging (advanced real-time video image processing), MicroPower Technologies (wireless surveillance) and Perminova (Web-based cardiology software).

EvoNexus is supported by local corporations including Qualcomm, LG, Nokia and Verizon. In return for their assistance, they get a more vibrant tech sector in their neighborhood - and they get a pipeline to new technology and talent.

A sponsorship costs $25,000 a year, which is a lot less than some companies pay to onboard a new app and a handful of programmers. (Hello, Facebook.)