Zynga is at it again. PetVille, its newest game, has launched in recent days. Like others in the genre, including Playfish‘s Pet Society, Slide‘s SuperPoke Pets and CrowdStar‘s new Happy Pets, the game revolves around cleaning, feeding, clothing your pet, and checking out your friends pets.

The bigger picture, of course, is that Zynga continues to expand into game genres that have already proven successful. Last month, it launched FishVille, a virtual aquarium game that competes against CrowdStar’s massive Happy Aquarium, and a range of other aquarium apps. Before that, Zynga launched Café World, which competes against Playfish’s virtual restaurant game Restaurant City. And, before that, it launched FarmVille — following a range of other virtual farming games.

All of these games have relatively similar dynamics, in a general sense, in that the object is social, and typically more collaborative than directly competitive. They all monetize through virtual goods, letting users obtain virtual currencies to use for buying virtual items via direct payments or by earning points through offers.

Zynga has the most muscle, though — it cross-promotes games in its gaming toolbar, for example, and spends heavily on Facebook advertising to get the game in front of more users. These and other factors have helped its games grow quickly. FishVille, for example, now has 22.4 million monthly active users, acording to AppData.

The pet genre is already pretty crowded, though, so this will be a big test of how well Zynga can grow versus a range of established competitors.

To dig deeper into the virtual goods market, check out our new report: Inside Virtual Goods: The US Virtual Goods Market 2009 – 2010.