In Pictures: Swiss Army smartphone - 7 tools your phone can replace - Slideshow

Just like "when all you've got is a hammer, every problem looks like a nail" (or "when your problem is a nail, every tool you've got looks like a hammer"), when you've got a smartphone, there's a good chance there's an smartphone app, case, or accessory for whatever you want to do. Not just the easy or obvious things, like turning your smartphone into a GPS, mirror, or mini-periscope, or providing damage protection. I'm talking about a wide, even seriously whacky range of tasks.
Handyman's toolkit Thanks to its built-in sensors -- typically including GPS, gyroscope, accelerometer, magnetometer, and microphone -- your smartphone can easily serve as a variety of tools for carpenters and other handypersons, like a level (bubble level bar, surface level or plumb-bob) and compass. And a smartphone doesn't even need sensors to be a display-based tool like a ruler or a protractor.
Microscope Some tasks call for a microscope -- like looking for clues, or checking watermarks on paper money. Sure, you could pack a USB microscope that uses your tablet or notebook as a display -- but for seriously mobile microscope users, there are magnifying attachments for your iPhone, Android phone, or other mobile devices. (Most of these comprise the actual microscope plus a device-specific case.)
Thermal imaging system Thermal imaging -- looking for "hot spots" or other temperature differences has many uses. Home energy auditors do this to see where you need more insulation, better windows, etc. Industrial sites use them to spot anomalies before problems develop. They're popular for security monitoring, gas leak detection, and with firefighters. But traditional thermal imaging systems haven't been cheap. FLIR One's case will let a smartphone act as a thermal imaging system, showing infrared, even merging it with regular camera views and other data.
Geiger counter Standalone Geiger counters can be had for $100-$300... or you can Geiger-accessorize your smartphone. Radiation Watch's Pocket Geiger peripheral offers a sub-$100 -- solution for iOS and Android users. Note: This detects only X-Rays and Gamma rays -- not Alpha or Beta radiation. You'll also need their smartphone app: the iOS free Pocket Geiger Counter Lite or $6.99 Pro, or the free Android Pocket Geiger app. For an even cheaper, although not quite as accurate, radiation-sensing iPhone 4 solution, consider the WikiSensor Dosimeter App (iOS, $0.99). (You'll also need something that can cover the camera to block out visible light -- like an inch of black electrical tape, or your thumb.
Tremor recorder Medical professionals are finding a growing number of ways to turn smartphones into inexpensive, portable alternatives to traditionally costlier, bulkier tools. Lift Lab's LiftPulse (Free, iOS) app gives doctors an easy, affordable way to measure and quantify "tremor" -- a common symptom of medical issues like Parkinson's Disease -- using the accelerometers in an iPhone.
Self-defense device Unless you're one of Edgar Rice Burroughs' four-armed Green Martians, or an octopus, juggling a phone and protective pepper spray when walking through unsafe areas can be difficult. Problem solved -- Spraytect's Self-Defense iPhone Case (for iPhone 4/4s, $39.95) includes a detachable pepper-spray cartridge holding a proprietary blend of habanero and other peppers. According to Spraytect, the range of a spray is 3 to 7 feet. (Note, the cartridges meet TSA checkable-luggage guidelines.)
Stun gun To protect yourself at closer-than-pepper-spraying distance, consider a Yellow Jacket stun gun case. According to Yellow Jacket's web site, the shock delivered is more of a "sting" than a "stun." And the case also provides up to a full battery recharge for your smartphone.