This advice is not meant to replace common sense, decision-making skills, and responsibility for your own actions and choices. I am not in any way liable if you kill yourself after reading something I've written. That being said, there's a lot of paranoia among rock climbers that needs to be dispelled, and a lot of unsafe practices that need to be stopped, so let's get started by busting some long-standing climbing myths!
Dropped gear is an inevitable part of climbing, and everyone knows that you should retire your gear if it gets dropped...right? The truth is, aluminum just isn't a material prone to the kind of microfractures that would be invisible to the naked eye. It's too ductile of a metal, which is exactly why it's perfect for climbing gear (along with its light weight). If you've ever seen a pull test, you know that carabiners actually stretch quite a bit before they ever break. Aluminum's inherent ductile nature prevents the danger of microfractures.
Even the climbing gear manufacturers say (and, more importantly, their lawyers let them say), if it appears okay upon visual inspection, it's probably just fine to use. Similarly, REI employees have done controlled pull tests of carabiners dropped from 30+ feet onto a concrete floor and found no strength difference when comparing the "microfractured" carabiners to a control group.
Of course, this is all still subject to common sense. If you drop a #6 cam off the top of El Cap (and somehow manage to find it after a 2,900 foot tumble), it's probably worth spending another $120 for the peace of mind in knowing that your gear/life is safe. If, on the other hand, you just dropped a locker from the top of your local 50 foot sport crag, you're probably safe keeping the 'biner and saving a few bucks—as long as it looks okay after a close inspection.