Using the crowdfunding money-raising model popularized in the tech start-up world, these initiatives allow people to pool donations of time and money for civic projects of their choosing, rather than wait for the work to be done by public employees — or not done at all.

“We want to make government more like a vending machine,” said Jordan Raynor, founder of Citizinvestor, which allows governments and community nonprofits to submit proposed public improvements that residents could choose to support.

In Philadelphia, for example, the Parks and Recreation Department is using Citizinvestor to raise $12,875 to plant trees across the city, while in Boston a nonprofit named Tech Goes Home is soliciting $6,480 to buy iPads for a class of blind ­students.

“For parks, pools, and playgrounds, giving citizens the power to say where they’re going to put [those] is very powerful,” Raynor said.

Citizinvestor’s initiatives in Boston dovetail with an effort by the Menino administration, dubbed the Office of New Urban Mechanics, to upgrade infrastructure for the digital age. Nigel Jacob, cochairman of the office, said models such as Citizinvestor are a natural extension of how governments have been operating for centuries.

“When you think of taxes, governments have been crowdfunded for a long time,” Jacob said. But what make sites such as Citizinvestor appealing in his world, he said, is that they identify additional sources of revenue for cities.

And supporters get a deep sense of ownership in the projects they support.

“We like to call them micro-philanthropists,” Raynor said. “We like to give the people the feeling that they’re modern-day Rockefellers, for $5 or $10.”

Some civic-funding sites give supporters perks to encourage participation: Neighbor.ly offered donors coffee mugs and posters, while Cambridge-based Cauzoom lets visitors support Little League baseball when they purchase gift cards for local businesses.

Not all efforts at tapping the kindness of crowds require money: Boston launched a crowdsourcing venture last year, Adopt-a-Hydrant, that asked for volunteers to shovel out hydrants in their neighborhoods during snowstorms.

And for some entrepreneurs, crowdfunding replaces traditional financial sources that may be disappearing. At Tech Goes Home, for example, the nonprofit’s executives began looking for other funding when a grant from a government broadband initiative began to wind down.

But putting projects out for public help can be risky, because people might not be interested in supporting them. With a day to hit its goal, the Philadelphia tree project had raised just over 13 percent of its goal and seemed unlikely to succeed.