There are a number of flashlights including surefire lights which drop into a "moon" mode. IMHO this is a pretty good inidicator as well as being a simple solution vs installing a voltage sence circuit and LED's to show voltage...
Yaesumofo
It would be quite useless. The only purpose it can serve is tell you when hit a lower voltage limit for the pack/cell so you can turn it off before you damage the rechargeable cells.
The voltage profile of NiMH, NiCd or lithium doesn't allow reliable indication of the state of charge.
The reason laptops and Sony cameras/camcorders can very accurately predict the state of charge is that it records when full charge is reached based on termination points and literally counts the amount of energy leaving the pack before you reach end of discharge termination.
It has algorithm to compensate for usable capacity change as a function of changing load, duration elapsed in storage, etc but the prediction is not perfect so you have to do a full charge/discharge once in a while, not to condition the pack, but to calibrate the meter.
As to why it isn't integrated to every device, cost is an obvious reason, but it's quite possible that it just isn't permissible due to patent issues.