if you haven't taken a look at the official blizzard armory recently, you should go check out your toon's page. there's a neat new feature just to the left of your character and underneath the "achievements" link. blizzard's officially added companions and mounts "tracking" to the armory!
here's my armory collection as an example: quintessence
i use the term "tracking" loosely mainly because unlike WarcraftPets.com, you can't select which ones you desire but have yet to collect. currently the armory will only show you which ones you've acquired and which ones you haven't added yet. also, unlike WarcraftPets, the armory doesn't show the companions that are no longer available or regional pets that you cannot collect.
however, it does have nifty roll over icons that shows the name of the pet plus its source. there's also an advanced filter option to help sort through all the pets. the advanced filter is lacking in some options such as the ability to sort by pet type though (*cough* WarcraftPets Bestiary *cough*). :P
although it's not quite as in depth or detailed as WarcraftPets, it's still a useful tool and it will come in handy should our favorite companion pet fansite ever go down for maintenance.