I need a second charger and a car charger for my Evo 3D and the same for my wife's Nexus S, but I don't want to pay exorbitant prices for them. Will I fry or otherwise damage the devices using something like this couple with a cheap microUSB to USB cable priced about the same? Will the devices complain about using "unapproved" chargers and refuse to charge like some iPhone models do with some 3rd party chargers?
USB, mini-USB, and micro-USB cables are standardized, the voltage and amperage are the same across all standard compliant USB chargers. As long as the charger is a standard, generic micro-USB charger then it should be alright.
That doesn't make it impossible for a manufacturer to create a non-standard plug that looks like a USB plug, but they're pretty much screwed up if they do so.
Micro-USB is standard size and pinout, so it will provide the power on the right pins no matter what charger that you plug into the device. Usually any charger that has the Micro-USB plug will work.
Additionally, in most modern batteries, there are a lot of charging safeties inside the device itself, including overvolting, undervolting, charging too quickly, charging when the battery is too hot or too cold, and reverse polarity. I would be surprised if it would be possible to buy a charger on the market today which would ruin a phone.
No it won't cause any damage. Any generic charger will do. I use a generic car charger like that all the time.
Whilst it won't cause an issue, I've noticed with my HTC Desire that non-official chargers are seen as "USB" chargers by my phone whereas the official chargers are seen to be "AC" types. This is significant because my phone will only draw 500mA in USB charger mode, but will happily pull 1A+ in AC mode.
It's all down to the USB Battery Charging Specification, which specifies that if the D+ and D- pins are shorted with a resistance of at most 200 Ω, additional current can be delivered. The short disables data transfer, but allows devices to detect the Dedicated Charging Port and allows very simple, high current chargers to be manufactured. The increased current (faster, 9 W charging) happens if both the host/hub and devices implement the new charging specification.