Hardware is mounting a comeback in Silicon Valley. 

Since its founding 10 years ago, startup accelerator Y Combinator has spawned now-well-known names including Airbnb, Dropbox, and gaming network Twitch. But at Y Combinator's most recent Demo Day, an event where companies incubated in the program are unveiled, hardware companies stole much of the spotlight usually reserved for Internet startups, Fortune reports.

This year Y Combinator had 20 hardware companies in its class, compared with the usual four or five. "We've seen entrepreneurs really start to build amazing companies in hardware," Luke Iseman, who heads up Y Combinator's hardware group, told Fortune. The co-founder of Edyn, a farm analytics company whose flagship product measures soil properties, Iseman was a member of the accelerator's summer program last year.

So which startups are helping put a shine back on hardware? Here are three promising Y Combinator companies cited by Fortune.

The San Francisco-based showerhead startup saves up to 70 percent of the water used by traditional showers, according to the company. In its recent Kickstarter campaign, Nebia set a fundraising target of $100,000 and ended up raising more than $3 million. The seven-person company, founded in 2010, counts Apple CEO Tim Cook as an investor.

Founded in India, Auro Robotics makes a self-driving golf cart that is currently being tested at the University of California's Santa Clara campus. The "autonomous shuttle" is intended for use at places such as theme parks, resorts, city centers, gated residential communities, and airports. The vehicle can reduce the cost of manually-driven golf carts and mini buses by up to 60 percent, according to the company, which recently relocated to Silicon Valley.

In July, drone company Flirtey beat Amazon to the punch by becoming the first company to make a government-approved commercial delivery. The Australia-based startup bills itself as the world's first autonomous aerial delivery company, and recently inked a deal with New Zealand's largest package delivery business. Flirtey was founded in 2013 and is partnering with McDonald's to pursue meal deliveries.