Place Kitten gives you place holder images that make you wonder if you should bother ever replacing them, like:
Cupcake Ipsum generates much better Lorem Ipsum text that your run-of-the-mill tools, like:
Cupcake ipsum dolor sit amet. Toffee I love cake I love gummi bears cotton candy I love cookie. Wafer dragée lemon drops jelly-o jelly I love lollipop.
Fruitcake lollipop sweet roll muffin caramels. Cake I love macaroon biscuit candy canes dessert pie. Sweet apple pie lollipop jelly beans cheesecake gummies biscuit. Wypas I love croissant macaroon halvah.
Sweet roll tart toffee lemon drops candy canes soufflé bonbon. Ice cream tart cupcake I love icing tootsie roll jelly. Soufflé biscuit topping topping caramels pudding sugar plum cheesecake.
Halvah ice cream macaroon lollipop donut. Dessert gingerbread toffee gummies I love gingerbread applicake. Icing marshmallow cupcake.
Topping jelly beans fruitcake tootsie roll. Faworki soufflé chocolate cake. Dessert sesame snaps biscuit tiramisu cookie I love sesame snaps.
Placehold.it is where placekitten.com got their idea, it is also useful, but not quite as fun
(via Chuck Rose)
[Update 2/29/12]
Adding also PLACESHEEN.COM, yow!
(via Bob Archer)