Spike got an EIDAP microchip (
www.eidap.com) during his neuter surgery.
I volunteer at the local humane society and there are so many animals that come in with no means of identifying the owners. Either they have escaped or gotten lost, and now there is no hope of tracking down their homes. There are many options available, and we always recommend using at least two so as to have a backup (microchip, tattoo, tags, etc.).
Any responsible pound, vet, or humane society will automatically check for a microchip if they find your dog. Spike also has his city registration tag, which is another form of identification. Here in Edmonton, if City Animal Control finds your dog and he is wearing his tags, they have a "free ride home" program where they bring him home free of charge.
It's not the most fun procedure for them to have done, but it only takes a second and it is relatively inexpensive. It can save all kinds of heartache if your dog ever does go missing!