The Sims: Unleashed is the sixth expansion in the series, and adds a ton of new features.
The most notable edition to The Sims is that of pets. Your Sim family may now adopt a pet from the local pet adoption center. You can adopt a cat, a dog, a lizard, a turtle, birds, parrots or fish. Dogs and cats are the most unique, as they count as a member of your family, needing certain stats to be happy, such as energy and bladder. You must provide bedding (or they'll sleep on the floor), litter box for cats (dogs do it anywhere) and food bowls, as well as toys to keep them happy.
You can train your pet to do tricks, and in leiu of another Sim, you can use your pet to improve your social score. Cats and dogs will also chase away skunks and other unwanted critters that stumble onto your lot.
The second main edition is gardening. You can now buy seeds at a garden store, and plant (and grow) your own crops. You can then use these crops to eat or to sell.
The neighborhood has been expanded, with a total of 40 lots allowed on one neighborhood. The original lot is there, plus thirty all new lots, which can be zoned either "commercial" or "residential".
The phone book has been expanded, so if you ever encountered the problem where you had so many friends some of them disappeared off the list, this solves it. You now have the option "Call Neighbor..." and when it pops up, you choose the last name of the family, then the person you'd like to call.
As in all expansions, there is 125+ new objects, new clothes and skins, and new wallpaper/floor tile combinations. There are also some new NPC characters, such as Animal Control and the Pet Trainer.