Who has time to make their own online signage in PhotoShop (or in my favorite image editor, Paint.net)? ImageChef will create a cute image for you with your own custom text blended into it.
To get an image, you don't even have to register. But if you do, ImageChef will also host the image for you. If you want the image on your cell phone, it will cost $1.99. My one complaint: all the images are square or in portrait orientation, so they won't work as blog or site banners. But it's probably entertaining for the kiddies.
There are even more custom graphic generators at CustomSignGenerator.com. Related: you can make pretty convincing warning-sign PDFs at Online-Sign.com.