So far, most casual multiplayer games were clones or simple incremental innovations of popular videogames (i.e.: Worms clones). But now we are starting to see unique multiplayer casual games with new themes and mechanics.
Juan: My pick is a game made by Three Rings Design called Corpse Craft. This is a multi-player game part of their Whirled service. In Corpse Craft, each player manages an army of zombies which goal is to tear down the opponent's HQ. You build zombies by combining substances, and you get these substances playing a collapse-style puzzle game.
The game looks simple but it requires a great deal of strategy, specially when it comes to timing. This is not a classic RTS style of game where whoever builds the most units wins. You have to be careful what you build and when. Corpse Craft has great character design too. Each zombie connotes really well what action it can do.
Nick: My choice for this category is LittleBigPlanet. A lot has been said about LittleBigPlanet in terms of its visual style and user-generated content, but I think it's multiplayer offering is at least as interesting to the development of casual games.
The core way of playing LittleBigPlanet is to play with multiple people on the same screen. The platforming gameplay encourages many people to tackle a level at the same time and either aid or grief each other. So the physics elements that platforming levels contain are designed that players can work together to utilize them.
This includes multiple players pushing blocks and balls to create steps, one player moving ahead to retrieve items for the rest of the group, or even the pure Gauntlet-style gameplay of lending a trailing player to keep the team moving forward.
There have been a few casual and core games from LittleBigPlanet to Rock Band that have been exploring how cooperative play can be used in a game that's not an FPS or a fantasy MMO. This is a field of game design that has a lot of future potential, and LittleBigPlanet deserves credit for making an early and strong step in that direction.
Other games worth checking out are: