1) Despite the wide variety of Hell notes available, there's very little interest in them among banknote collectors. They've yet to be documented in any properly organised catalogue, which may explain why they're still so cheap to buy.
2) Exchange of e-mails, June 2013. Joel Anderson's site for collectors of coins and banknotes is here: http://www.joelscoins.com/.
3) I suspect it's the notes' badass name which appeals to the good folks at Planet Voodoo (no relation). They suggest adding Hell notes to mojo bags in order to draw money to you, using them as "bribes to the Dark Spirits" when practicing necromancy or incorporating them into your crossroads or graveyard rituals: http://www.planetvoodoo.com/voodoo-temple/hell-bank-note-ritual.htm.
4) The Chinese folk healer called a hoat-su uses Hell notes in his own magic rituals too. He stamps the notes with characters describing the problem at hand, then burns them and stirs the ashes into a cup of water. When he's taken a sip of the water and spat it out again, the problem's gone too.
5) Hell notes can also be burned at the Hungry Ghosts Festival, when the dead return to visit their living relatives. This is thought to appease the ghosts with no families of their own, who would cause mischief otherwise. Capturing Penang has this photo: http://www.capturingpenang.com/2010/08/hungry-ghosts-returning-to-hell.html.
6) Diyu translates as something like "underground hold" or "underground court". It's both the afterworld's holding cell and its courtroom.
7) According to Wikipedia, over 450m people around the globe practice Chinese folk religion, accounting for about 6.6% of the world's population. China Consumers' Association figures reported here say the nation spends over ¥10bn (about $1.6bn) of real money on paper funeral goods every year. The most expensive single item CCA found was an elaborate paper villa priced at ¥16,888 (about $2,700).
8) Other creatures you'll see on Hell notes include the foo dogs whose statues guard public buildings in China and the ch'i lin, a hybrid creature from myth which is said to appear when a great leader dies.
9) Joel Anderson told me the only other currency he's ever seen used on a Hell note besides yuan and dollars is the Vietnamese dong. Many of the earliest Hell notes seen in the USA were brought home by soldiers who'd fought in the Vietnam War.
10) As with any other TV show, this programme's sponsors were keen to see their company name included in the show's title. Rather wonderfully, this has led to the show being called Mongolian Cow Sour Milk Supergirl.
11) China Daily, April 25, 2006.
12) Daily Telegraph, March 22, 2007.
13) Burning real money, even in small denominations, is considered bad luck in China.
14) In still another tradition, Hell notes can be piled neatly on to a relative's grave and left there intact as a simple offering. A photo appears on Joann Pittman's blog page below.
15) Outside-In, by Joann Pittman: http://joannpittman.com/chinese-culture/2012/hell-money/
16) ) I've also read accounts of mourners who like to retain one Hell note from every funeral they attend, presumably as a remembrance of dead friends. I've no idea if this is a widespread custom or not.
17) This interview was conducted by students at the University of California in Irvine. You can find their full report here: http://www.anthropology.uci.edu.
18) It's official: the Devil really does wear Prada!
19) The X-Files: Hell Money, written by Jeffrey Vlaming (Fox). Season 3, episode 19. First aired: March 29, 1996.
20) I put this idea to the British Taoist Association. "I rather like your suggestion," Shidao replied. "Pagoda-style structures are often used to mark a significant location and provide a place for seeing further into the distance - surely good attributes for a wang xiang tai."