If you designed the perfect little rectangle, would you bristle if companies started designing little form-interrupting gewgaws to slap onto it? I guess that's the price you pay for ubiquity. And you can't deny that some of these widgets add good functionality, like the Glif.
And while it looks a bit absurd, PhotoJojo's latest $35 iPhone add-on is a telephoto lens. It gives you 8x zoom, features a manual focus on the lens body, and comes with a stabilizing tripod.
Prior to the telephoto lens, PhotoJojo kicked off the camera-add-on thing with their FishEye, Macro and Wide Angle cameraphone lenses:
More recently, Square made the credit-card slider add-on, turning you into a self-contained retail operation:
The GoBiz i-Microphone runs $25 and will reportedly boost recordings by 12 decibels. Plugs right into the headphone jack and uses RF shielding to block interference and feedback: