List the pet on the following websites, put up flyers around your
neighborhood, and call local shelters to report the pet you found:

http://www.pets911.com
http://www.craigslist.com
http://www.petfinder.com (classifieds)
http://www.petharbor.com
http://www.lostpetsos.org/
http://www.fidofinder.com  (for dogs)
http://www.tabbytracker.com (for cats)
http://www.missingpet.net
http://www.lostmydoggie.com

It is strongly recommend that you take the pet to any veterinarian (or
animal shelter) and ask them to scan for a microchip.  Vets will usually
perform this service free of charge and if a microchip is found, it is the
quickest way to locate the owner.

Also, contact your local newspaper and ask them to run a free "found" ad,
since most papers will do this as a free service to the community.

While many people are reluctant to bring a lost pet to the animal shelter,
often it's the best way for an owner to find their missing pet.  That's
usually the first place they'll look.  If you can't bring the pet to the
shelter, at least contact them to ask if they will allow you to post the
pet on their website's lost & found, and/or if they have a log they keep
for people who call in to report lost and found pets.