HTC mains-to-USB charger: use to charge other non-HTC but USB-chargeable devices?
I treat chargers with the same specs as interchangeable... many USB chargers are rated as 5v 500mA (or .5A); in most cases, you should be able to interchange them (it may take excellent eyesight to actually read the specs; on my Motoroloa chargers, they are very faintly engraved).   Some Apple chargers produce more current than the standard 500mA, so I would not use them to charge products other than the intended.   Since most computer USB ports are rated for 500mA, chargers with the same rating should charge any USB-chargeable device without problems.