Requires iOS 4.2 or higher
Allows you to have timestamps on photos inside camera.app. You can take a photo with the normal camera app, and will have two photos taken, one, which is the original, and one with the timestamp. (Of slightly lower resolution) Easily customizable from settings, you can adjust the font size. The font color, even have custom text before the time stamp. You can have a 24 hour time if you'd like, and the time stamp also includes the date. Tested and completely works on 4.2+ (including Verizon iPhone).
No new icons are added to your home screen. Tweak options can be configured from Settings.app
See further below for Screenshots