I have come across some species of living organisms who are able to emit light at whim. Can that ability be incorporated into a portion of human skin (a specialized tissue)?
Genetic modification of any cell to overexpress a fluorescent protein will not 'generate' light, per-say: fluorophores absorb particular wavelengths and re-emit in less energetic wavelengths, dispersing some energy as heat. (Perhaps there is some semantic choice here, but there is no transduction to light from another form of energy.)
Bio-luminescence, on the other hand, can be considered generation of light. There are proteins commonly found in marine animals (which skin cells could be engineered to express) which convert chemical energy to emit light.
With regards to generation of electricity, I think this gets a bit interesting...
The electron transport chain (ETC) in the inner membranes of our mitochondria carry out a series of redox reactions which serve to oxidise food metabolites (thus freeing electrons). In order to maintain the free energy gradient that favours this directional cascade of reactions, electrons and protons are finally accepted by a continual supply of oxygen atoms, producing water. (You may be aware that aerobic respiration consumes oxygen and produces water.) In nature (in aerobes), oxygen acts as the final electron acceptor, or an electron sink.
Bacteria also have electron transport chains for their respiration (mitochondria were once bacteria, after all), and people have wondered whether these redox reactions could be used to produce a current instead of reducing oxygen to water as the final step. We normally use current in the form of electrons moving along a metal wire.
This is the idea of a "microbial fuel cell". Just like a battery produces a current from a chemical redox reaction, the ETC might be recruited in to producing a current from its chemical redox reactions.
I think this is a very difficult piece of bioengineering, since the ETC is integrated in to respiration and needs to be de- & re-coupled to some kind of inorganic mediator and ultimately a circuit; and also because the ETC produces very low voltages, I think.
There is a nice wikipedia page on microbial fuel cells that you might find interesting, if you think this is relevant to your question.
Sort of problematic is the "at a whim" part. Just a brief search to wikipedia on electric organs, here, we see that the electrocytes are controlled by a nucleus of pacemaker neurons. This is all unique to electric fish, and you'd have to express that whole system in a human being to elicit the same effect. You'd also have to consider how your organism is engineered to withstand the actual electric shock emitted by the electric organ.
We can already tag molecules with luciferase to emit light as a mode of bio-luminescence. To get an appreciable reaction, or rather an appreciably bright light, I can imagine we'd need a ridiculous amount of luciferin in vivo, though. We also get stuck at the part about it being on a whim. The control mechanisms/structures for the light-emitting reaction simply aren't present.
Yes, the fibroblasts of skin cells can quite easily be Transfected with a fluorescent gene such as GFP. Fluorescence doesn't actually produce light, the fluorescent molecule just changes the wavelength of light shining on it.
Fibroblasts can also be made luminescent (light producing) by acquiring genes for firefly luciferase or aquoren from jellyfish. This is a little more tricky since luminescent molecules typically require cofactors such as ATP, Calcium or other proteins.
Both methodologies are readily performed in vitro.