The LA Times ran an interesting article about how homemade (rather, prisonmade) greeting cards provide investigators with insight into gang activity behind bars.
"That's where you get a lot of your information, from these birthday cards," said Officer Steve Preciado, a Lancaster gang investigator. "A lot of times their family members won't send them nothing. But the gangsters will put their nicknames on these cards, and where they're from, like 'Shorty from Pacoima.' So your job is to find out who Shorty is."
Link (Thanks, Gabe!)