One item that I've found to be extremely useful in live sound reinforcement is the Whirlwind Qbox. It's an audio testing and utility tool that does several things - cable/line tester, mic, tone generator, and monitor. I use it most of the time to check mic lines on stage by sending the 1khz tone down a line back to the mixer to confirm that it's working. You can also switch it over to the mic and speak into the channel. If you have two Qboxes (often the case when another audio engineer carries one with them), you can have an intercom between two points!The Qbox also has the ability to detect phantom power, which is a quick and dirty way to double check if a line is connected to the mixer. The monitoring mode allows me to check outputs from mixers, guitars, keyboards and dynamic mics; there is a little loudspeaker, although small, but allows for basic checks. If you need to hear a little bit better, then just plug in your headphones into the headphone socket. This beats having to run back and forth between the stage and console, checking for the signal, especially if you're working alone. Did I also mention the ability to monitor a three wire production intercom?
There are other different types of testers out in the market, but I reckon the Qbox is generally one of the best mic/line tester out there since its debut in 1997.
