There are countless websites on which lost dog ads are posted, Craigslist being probably the best local source. Other examples are Fido Finder, Find Toto, Lost My Doggie, Pets911, Petfinder Classifieds, and there are a ton more.
Then to find found dogs to match them to, many of the same sites also post found dogs. However, there are always fewer found dogs, partly because it's not that common for people to go to the effort of searching for a lost dog's owner, and because typically, a person who happens to be in a position to rescue an at large dog in need isn't necessarily in a position to foster that dog until the owner is found. So, finding shelter websites that post dogs is generally a good way to make matches. Pet Harbor is the best site I've found for this.
If you think that people don't need help finding their dog at the shelter, think again! Many people don't think of the shelter, don't check often enough, don't know how to check online. Then there's a problem that this guy ran into -- his dog was taken to a shelter that assigns names to stray dogs, from the time they are admitted. This guy may never have gotten his dog back if there wasn't someone to explain to him that his dog was the one on the Pet Harbor website that had been immediately given the name "Tangerine".
