It's D-day in Mountain View, California. Demo day, that is.

Silicon Valley incubator Y Combinator hosted its semiannual start-up coming-out party Tuesday at the Computer History Museum.

This graduating class is smaller than some of its predecessors--comprising only 47 start-ups compared to the fall class of 75--due to more rigorous vetting on the part of YC administrators, says founder Paul Graham.

"Hardly any start-ups in this batch are bad," he announced at the event.

The 500 or so investors in attendance seemed to take the tongue-in-cheek recommendation to heart, milling around the second story of the museum during presentation breaks to question founders, collect cards, and shake hands--presumably making a few informal deals in the process.

Here are four start-ups--and one non-profit--Inc. spotted from Y Combinator's spring class that you may want to keep your eye on.

Thalmic Labs

Thalmic Labs is a hardware company based in Kitchner-Waterloo, Canada. Its claim to Y Combinator fame? Myo, a wearable arm band that detects muscle movement and lets you use gestures to control your computing devices. Imagine playing video games and flipping through presentations with just a flip of the arm.

According to co-founder Stephen Lake, Myo is the next generation of computer hardware. Unlike other gesture-control technology, such as Microsoft's Kinect, Myo doesn't require cameras to work. "And we've patented the heck out of it," he adds. The device sells for $149 and will start shipping in late 2013.

Microryza

Inc. recently featured this so-called "Kickstarter for science," in a round-up of niche crowdfunding sites. Co-founders Cindy Wu and Denny Luan came up with the idea for Microryza when the pair, both 19 years old at the time, developed a treatment for Anthrax--but couldn't get access to funding to pursue their research. 

The current research funding model only applies to tenured professors and mid-life professionals, Wu says. To Microryza's founders, this didn't seemed fair. The company, named for a symbiotic fungi that stabilizes plant soil and fends off pathogens, exists to make the research funding process easier. According to Wu, the platform has helped to fund over a dozen independent science research projects since its launch last April. Like Kickstarter, the crowdfunding platform makes its money by taking a small cut of the total amount raised by each funded project. But, as Wu puts it, both she and the project backers "do it for science."

Watsi

Watsi doesn't fit Y Combinator's typical alumni profile--mainly because it's not out to make a profit. Watsi is the incubator's first ever non-profit, and it functions as a crowdfunding platform for low-cost medical care in developing countries. Donors can go on the site and fund an individuals' treatment. Watsi sends email updates about the outcome of the person's care. Separate donations fund the site's operational costs. 

Graham explained the unusual presence of a non-profit in Y Combinator's midst with his opening comments: "We have one non-profit--one intentional non-profit, I mean--that is looking for donations [instead of funding]."

Co-founder Chase Adams says of Watsi's journey: "Up until today, it's been pretty much the same as a start-up."

PayTango

"Biometrics are the future," says Umang Patel, co-founder of PayTango, which links users' fingerprints with their credit card information. The company uses a biometric scanner and its back-end software system to process payments, and makes money through merchant partnerships. The system has been rolled out to the Carnegie Mellon campus and surrounding merchants.

But Patel is dreaming big. Imagine the possibilities of a more secure identification system, he says. Patel thinks that fingerprint recognition could revolutionize your entire wallet. Everything from credit cards and IDs to bus passes and airplane tickets could be literally at your fingertips, he says. PayTango wants to raise seed round funding and make its devices more widely available. College campuses are an obvious focus, says co-founder Brian Groudan, as are gyms and "places where you carry your life on a card."

Goldbely

You know those Harry & David's pears you send to your in-laws each year? Well, Goldybely thinks you can do better. CEO Joe Ariel wants to give food delivery an upgrade by shipping gourmet eats from anywhere to anywhere--in the United States, at least.

Craving a sandwich from Katz Deli in New York, but stuck in Portland, Oregon? Goldbely will pack it so it doesn't spoil and ship it to your door, says Ariel, who cut his teeth in the food delivery business as CEO of Delivery.com. Delivery can take anywhere from 24 hours to a few days to arrive.

The goal is to make "legendary" local good available in any locale. "We're here for the mom-and-pop shops that don't know how to market online," says Ariel, who adds that Goldbely has no intention of going head-to-head with mail-order giants like Omaha Steaks or Harry and David's.

Goldbely plans to make money through merchant partnerships. According to CTO Trevor Stow, the company relies on customer and staff recommendations to bring iconic eateries on board--and they're open to suggestions.

The company wrapped up its pitch with a self-catered buffet featuring sweet treats from a few of its clients, including Jeni's Splendid Ice Creams, an artisan ice cream parlor in Columbus, Ohio. Maybe the way to a VC's heart really is through his stomach?