The American Society for the Prevention of Cruelty to Animals (ASPCA) has some tips for what to do if you find a stray cat in your neighborhood. 
Rena Lafaille, the director of administration for the non-profit organization, said that kittens tend to pop up during the spring and summer during the breeding season, and the newborn animals are at their most vulnerable, in need of around the clock care. 
First, make sure the kitten isn't injured. If it is, contact a local vet or shelter. 
If it isn't, try to determine how old it is. If the kitten is younger than two months, its mother might still be around to take care of it, and the best course of action is to monitor the situation until you've determined if the kitten is an orphan. 
Orphan kittens under two months of age need constant supervision and care, so if it looks like the mother is no longer around, again contact a local vet or shelter. 
If it's an older cat, it may be a so-called "community cat" and might be perfectly fine left alone. However, the ASPCA does recommend making sure those cats are spayed or neutered and vaccinated against rabies to keep the local cat population in check. 
Fostering, of course, is an option as well, and may be exactly what a cat or kitten needs. 
"Fostering is an amazing opportunity to help out a shelter just because you're freeing up space in the shelter for another animal in need," Lafaille said. 
More details about what to do are available at the ASPCA website