Rainbows abound in mythology. In Greek mythology, the messenger Iris travels on the rainbow between gods and mortals. In Judeo-Christian stories, the rainbow is the sign of the covenant between God and life on earth, sent first to Noah after the Flood.1 An old mariner saying uses the rainbow as a weather guide: "A rainbow in the morning is the sailor's warning; a rainbow at night is the sailor's delight." Historical traditions from many countries around the world refer to the rainbow as a path, a bridge, a road - to God, to heaven, the afterlife. If you saw the ridiculously good-looking Chris Hemsworth in this year's Thor, you'd know that the Norse gods refer to the Bifrost, a rainbow bridge that leads from Asgard to the portal that sends them careening through the nine realms. And of course, the Irish have their leprechauns guarding pots of gold at the end of the rainbow.2
Not everyone wants to sing songs about rainbows - there are a few cultures for whom rainbows are harbingers of bad stuff to come. The rainbows were bad luck omens for Japanese, as it reminded them of the evil snake. Rainbows sent people of Nicaragua and Honduras scurrying to their huts to hide until it passed, as it was a sign of the devil. I also found reference to an old Slavic tale of a mortal touching a rainbow and turning into a demonic creature thanks to the god of lightning and thunder.
In the 1939, Judy Garland sang "Somewhere Over the Rainbow," which holds the top spot in the RIAA and NEA list of songs of the century.3 Kermit the Frog first performed "Rainbow Connection" in 1979. In the 1980s there was a cartoon called Rainbow Brite, a girl who, along with the Color Kids, her sprite Twink, and white horse Starlite, live in Rainbow Land and are in charge of keeping Earth awash in colors.4 And there are plenty of cultures and movements that use the rainbow as a flag - gay pride is probably the most well-known, but a rainbow flag has also been used to symbolize peace and diversity.
Not bad for what Wikipedia calls an optical and meteorological phenomenon.
1 See Genesis 8:9-17
2 Or, perhaps, lucky charms.
3 Fun little tidbit - the song almost didn't make it into the film as the MGM chief exec and a producer thought it slowed the movie down, but an associate producer and Judy Garland's vocal coach kept after them, and the song made the final cut.
4 I was Rainbow Brite for Halloween when I was a wee lass, and I had her doll and the plush Twink. If I knew where that photo was, I'd put it up.