After reading Tom's description of PopCap's Plants Vs. Zombies on Offworld, I downloaded it and played it with my six-year-old daughter over the weekend and last night.
The game is a classic "tower defense" game. Brain-eating Zombies on the street are shuffling towards your house. The only way to defend your family against the rotting cannibalistic invaders is by sowing seeds of different species of fast-growing plants designed to stop or slow them down.
At this point, everyone else in the house is sick of Jane and I talking about how much fun Plants Vs. Zombies is and what we'll need to do keep the zombies from overtaking us when we play again (as soon as she gets back from Kindergarten class today).