For most applications a good quality Digital Multimeter (DMM) is what you need. These can run from $6 at Harbor Freight to $100s for the really high-end meters. If you are going to be using it with low voltage stuff and you don't need fast response time or really tight precision, go with a less costly unit. I have one of the $6 HF DMMs and its good for fast continuity checks wall wart outputs, and so on, but I also have a $200 Fluke 175 that I use for AC checks and signal monitoring. You can spend a lot more than that, too. If all you need is simple continuity and low voltage power checks, go cheap, but I'd never use the $6 meter on AC line voltage.