petfinder is a great resource if you are wanting a pet. It has a good database of rescue groups and shelters. It has good name recognition aparently. fidofinder is also doing a good service. Like many of the sites out there they push registering your dog with them. That can work but if I remember right it was pretty dependent
on zip-code. Many of their options are fee based such as uploading a photo, length of time your missing dog is posted is based upon the reward amount you are offering of which fidofinder gets a modest cut upon being reunited. This dollar amount is public and grants you more exposure on their website. Right or wrong, most sites warn against publishing a dollar amount. Only recently did fidofinder include the pet's gender in the listings (thanks to me!

) but it could be more helpful as a search option. Their thought was a person missing a pug would check out all pugs. Probably. It makes it easier and more likely for someone finding a dog to get involved and stay involved.
petdetective in my oppinion has the most user friendly set-up. It has a very easy to use (and thorough) search. Their alert options(fee-based) are noteworthy. Though they rely on shelters, vets, or rescue groups registering their organizations, it seems to be the only pro-active measure out there as far as looking for your lost dog is concerned. I have had some contacts from rescue groups trying to reunite me and my missing dog through this site as well as being notified when ever a possible match has been posted.The staff sent me a listing of known organizations in my state who were not registered with them.
K911.com is another site but I'm less familiar with them. I'm registered, and like most of the good sites they offer a secure method of being contacted, just haven't found their privacy option they tout yet. This is a plus with any site as I have been spammed without mercy. There are a lot of sites trying various ways to help and when you are missing a dear friend, you will take advantage of any help offered.
I had started a thread earlier under rescues regarding help sites. There are many. That can be seen as good or inefficient. My searches for those sites led me to this forum as I am sure it will others, so I think it is very important to get the names of the ones who do it well out there. When someone finds a dog they will know where to go.