Although the they're not as sophisticated as the Flash greeting cards you can now get, I've always liked the simplicity of Apple's iCards. With PostCard you can now make your own iCard style postcards with your own images. You just drag the image onto the card, adjust the size, write your text and pick your stamp. If you so desire you can even customize the stamp image and postmark text. As you can see above there's a convenient button for copying it to the Clipboard for pasting in your email application.
PostCard is $10 and a demo is available (though as you can see, it will watermark the image until you register).
[Via Hawk Wings]