The rainbow flag is, of course, most noticeably used by gay people (six-colour version). A seven-colour version, sometimes with PACE written on it, is often used by peace campaigners. The year I started college, Trinity LGBT society had their stand next to the Socialist Workers Party; both had a rainbow flag. Apple computer used to use an apple-shaped one. (It is often suggested that this is because Alan Turing, noted computer pioneer and gay, killed himself using a cyanide-laced apple, a la Snow White. There doesn't seem to be any evidence for this, though, and it is surely a bit macabre?) And so on.
Apparently, the 'International Order of the Rainbow for Girls', a sort of Masonic Girl Guides, used to use one too; they had to change it because it was 'stolen'. The author of that comment seems rather pissed off about it, but really, the Freemasons are worried about the effect the scary gays (, socialists, iPods, et al.) are having on their reputation? Ask the average person on the street which they find more frightening.
Go on. Now. That's your homework for this evening.