My current favourite piece of test equipment is the humble Amprobe AM-500 multimeter. Its been a constant companion in diagnosing issues with circuits and spends most of its life checking cable assembly continuities.
It's not, however, the tool for decoding the signals sent across IR data links (nor analog signals). Apart from providing simple control options for small props, I also would like to build an open source laser tag system. Currently, nothing on the market really hits all the features I would like. Being able to see the difference between the transmitted and received packets on one screen would be a huge boon.