With California's IOUs as its newpeg, The Wall Street Journal looks back at the alternative local currencies—some private, some public—that flourished during the depression of the '30s. Be sure to check out the slideshow that goes with the story; it's filled with colorful scrips like this western Michigan money:
The IOUs in California are rather different: Not only are they not as pretty but, as Kenneth Anderson notes, they "they are made out to particular individuals for particular amounts—they physically resemble checks." There have been informal and formal efforts to set up secondary markets for the IOUs, but I wouldn't expect that paper to become anything akin to a circulating currency.
[Hat tip: Bryan Alexander.]