On land you don't see glowing life forms a whole lot. In the sea, they're extremely common; and the deeper you go, the more the glow. Take the interesting case of the cookiecutter shark, who (to quote Wikipedia's page on the subject) "uses bioluminescence for camouflage, but a small patch on its underbelly remains dark and appears as a small fish to large predatory fish like tuna and mackerel. When these fish try to consume the 'small fish', they are bitten by the shark." Clever!
Several jellies ("jellyfish") glow too, as do a number of squids, marine worms, and fish. The Bioluminescence page at UC Santa Barbara has some gorgeous photos. Harbor Branch Oceanographic Institution's Bioluminescence page has an extensive and quite interesting Q&A on the topic, which includes the intriguing finding that "Bioluminescence has apparently evolved many (possibly as many as 30) different times in evolutionary history" - that is, separately and unconnected from each other - suggesting it is a very useful feature indeed.
Fireflies, which I'd love to see but still haven't, are probably the most famous example of bioluminescence, as you don't have to go into the deep sea to find them (but you may have to go into the deep South). They luminesce as a communication device, to find and attract mates. And in a page right out of Alice in Wonderland, a fungus that grows in forests sometimes makes decaying trees glow faintly at night. That phenomenon is called "foxfire," or occasionally "will o' the wisp" (though I think that term can be attributed to marsh gas flames too). I clearly need to spend more time in forests at night, as I've never seen foxfire either, and it sounds awesome. (Linguistic note: some sources claim that the word "foxfire" comes from "'faux' [Fr.: 'false'] fire", and has nothing to do with foxes. I'll buy it for now.)
But the coolness doesn't stop there. Chemicals of bioluminescence have already been isolated and used as tracers for certain medical and biotechnological studies; and (according to Wikipedia) engineered bioluminescence is being considered for ideas like:
- Christmas trees that glow by themselves
- roadside trees that do likewise (saving electricity costs)
- crops that luminesce when they need water
- novelty pets that bioluminesce, such as rabbits or mice
Why stop there? Think how much some people would pay to have bioluminescent hair or tattoos. There's a fortune to be made here; and, I suspect, a sci-fi novel somewhere in the makings too.