The mnemonic I learned in middle school for the categories in the electromagnetic spectrum is Raging Martians Invaded ROY G. BIV Using X-rays and Gamma rays.
 
I always thought the last two pieces were silly--what's the point in making up a mnemonic if it's identical to the thing you're trying to memorize? Additionally, the "Invaded" part makes it sound like the Raging Martians are performing an unwanted medical procedure. I like the "Raging Martians" bit, though.
 
I've recently come to prefer
Raging Martians Insulted ROY G. BIV, Urge Xenophobic Giraffes.
 
That's totally a headline.
 
New contest! Come up with the best (read: wittiest) EM spectrum mnemonic. There's no limit to the number of suggestions by one person, as long as they're all original. Extra points will be awarded to those who devise alternatives to the awkward BIV surname (ROY sounds like a Duck Dodgers-style astronaut, so it's aesthetically unjarring). The winner, as judged by the impeccable taste of yours truly, will get some sort of trinket, specifics to be announced when I've thought of something suitable.