Plant Nanny is fun, little app that combines the process of growing plants with the healthy habit of drinking water. It's an unusual combination, but the app pulls it off wonderfully.
Plant Nanny lets you pick a cartoon seedling and plant it in a pot on your virtual window sill. As you drink water throughout the day, you add those glasses to the app and water your plant. The more water you give your plant, the faster it will grow. When it is fully grown, you can move it outside and start over with a new seedling.
The app encourages you to keep drinking by rewarding you with seeds from your fully grown (and watered) plants. These seeds can be used to buy more exotic plants besides the dandelion that is free with the app. Plant Nanny also supports notifications, which can be setup to remind you to drink your water at a regular interval.
Plant Nanny is available for free from the iOS App Store.