You get the skunk pet Stinker for meeting the 50 pets achievement (changed from 75), and it's a good idea to start collecting now, before those rare and cross-faction pets really skyrocket in price. And if you're worried about your bag space, have a Pet-Holding alt to send the bind-on-use pets to until the new pets-as-spells feature is implemented. That's what I'm doing.
Check WarcraftPets.com news regularly to see if any new easy-to-snag pets have been added to the game. (Right now, there's a limited-time Spirit of Competition for winning battlegrounds.)
Keep track of your pet collection through WarcraftPets. This one's mine. (Note I excluded the cockroach. *shudder* I will never get that one, not even for a cute prize.)
More pets will come in Wrath, so don't worry if you don't have a lot of time for the hard-to-get ones. Just take advantage of the easy yearly events that include pet rewards, such as Children's Week.
Cross-faction pets can be found on the AH, particularly the neutral AH. You just have to wait for someone to post them.
- H-Eversong: Jilanne (gold, red, silver dragonhawk)
- H-Orgrimmar: Xan'tish (black, brown, crimson snake)
- H-Undercity: Jeremiah (cockroach) (my post)
- H-Thunder Bluff: Halpa (prairie dog) (my post)
- A-Exodar: Sixx (blue, white, yellow moth)
- A-Elwynn: Donni (bombay, cornish, orange, silver cat) (my post)
- A-Darnassus: Shylenai (brown, black hawk)
- A-Stormwind: Lil Timmy (white kitten) (my post)
- A-Dun Morogh: Yarlyn Amberstill (snowshoe rabbit) (my post)
- Both-Booty Bay: Narkk (cockatiel, senegal parrot)
- Both-Shimmering Flats: Magus Tirth (ancona chicken)
- Both-Outlands: Dealer Rashaad (various pets, some new colors)
- Cluck! (prairie chicken) (my post) (Horde tutorial)
- Alliance Only Newest Member of the Family (sprite darter)
- Kibler's Exotic Pets (worg pup) (my post)
- En-Ay-Es-Tee-Why (spider) (my post)
- Skywing (miniwing)
- OOX Escort Feralas/Tanaris/Hinterlands (mechanical chicken)
- Spawn of Jubjub (orange-blue frog)