DSLR cameras are great for taking the best pictures you possibly can, though they're worse than point-and-shoots and cameraphones in one key feature: keeping a low profile. Pointing a bulky camera at someone pretty much guarantees they'll know you're taking a picture of them, which is typically OK, but in some select instances is not.
It's for those instances that the Super-Secret Spy Lens was made. Attaching to the end of your camera lens, the accessory has a cutout with a mirror inside, letting you aim your camera way over yonder while secret snapping pics of targets to your right or left (or even above or below). It's said to fit all common SLR cameras, though it won't work with fish-eye or wide-angle lenses. Not bad for $50.