The Undermine Journal is finally showing pet items AND caged pets! Yaay. :) Now you can check up on pet prices on the AH across multiple servers without having to log into each realm.
For caged pets, the website shows what breed it is, but it doesn't list the pet's level or its quality. So while it's convenient to check up on whether a certain pet's available on a server, be sure to double check that the pet is what you're really looking for before throwing your money at the auctioneer.
In addition to showing caged pets, if you're on a PVE server and don't have the time to camp out at the BMAH to check for especially rare pets, The Undermine Journal does it for you! It's only for PVE servers, though, because apparently they gather the information manually. WowInsider covers how they do it.
So if you're big on selling/buying pets from the AH, The Undermine Journal is a very useful tool and resource. Heck, I don't play the market much but I still enjoy checking out what's for sale and how much pets/items are going for. :P