WoWScan.com is a really awesome website/tool for pet, mount -and recipe collectors.
The website lists the cheapest items on every server with a single click on the search button. This is especially useful to (TCG) collectors who are willing to transfer to another server to get some of the cheapest deals possible. It's of course also useful as a price guide for items, as it gives you a quick idea of how much an item is worth.
WoWScan.com is planning to expand the website to include every in-game item possible, but for now they've launched with pets, mounts and recipes only.
Their website is 100% legit and free to use, no user accounts are required!