I highly recommend microchipping your cats, even if they are indoor
only. Emergencies can happen and they can get out. If you heard any of
the Katrina stories, pets who were chipped were much more likely to be
reunited with their owners.

I like Avis chips. You pay a one-time fee to get registered (entered
into their database). You don't need to pay again unless there is a
change (address, owner, etc.). Some other brands charge an annual fee.
Avis is commonly used at vets and animal shelters, so should your pet
get lost the chip will be easily recognized.

Rene