iPhone/iPod touch: Small sensors in smartphone cameras always lead to everything being in focus. SynthCam uses some clever tricks to allow your iDevice to emulate the shallow depth of field look you can achieve with DSLR cameras.
SynthCam works by using your iDevice's video recording functionality to record multiple frames and use the combined images to create a fake shallow depth of field in the out-of-focus areas of your photograph. You set a (relatively small) focal point when you begin using SynthCam, move the camera left, right, up, and down for a few seconds, stop recording, and you'll end up with a composite image that emulates shallow depth of field. You can see an example photo of my car keys and sunglasses to the right.
Because SynthCam uses video to create this effect, images are going to be low resolution. If you have an HD camera on your iDevice, however, you do have the option of creating somewhat higher resolution images. While it had some difficulty tracking darker subjects, that seems to be more of a limitation of the hardware than of SynthCam's abilities. Overall, if you want to create the look of shallow depth of field without breaking out Photoshop, it's a fun option.
SynthCam is available for $1.19, right now, in the iTunes App Store.