(In three days.)
In terms of folklore, American Halloween--despite claims that it is tied to ancient Celtic and medieval European festivals--is a very recent invention.
Around Victorian times, Halloween in the United States began to involve private parties which would include fortune telling and apple bobbing. Themed costumes entered the picture in the 1900s, but costumes--and for that matter, begging--were associated more with Thanksgiving than with Halloween.
The link of Halloween to costumes and, specifically, to trick-or-treating by children didn't take place until the WWII era. The idea was to reduce adult pranking and general out-of-handness by concentrating on the child aspects of the holiday.
I grew up at the tail-end of this children's-oriented paradise. I remember going trick or treating, carrying pillow cases (none of those cutsy jack o'lantern shaped "bags"--those things aren't even close to big enough!), and I have a vivid recollection of my brothers discussing whether or not to go home and get my little red wagon, so (1) I wouldn't get tired; (2) there would be more room for the candy. When we did get home, we would count, categorize, and trade our candy. (Then, as now, I preferred chocolate to hard candy and just about anything to Baby Ruths.)
And nobody checked it. In fact, not only did nobody check our candy, people gave away seriously uncheckable stuff: soda, homemade caramel popcorn balls. I grew up in pure suburbia, and there were tons of kids on the street and tons of houses ready and waiting to hand out treats.
And then the scares started. Interestingly enough, the big scare--the Candy Man--happened earlier than I had remembered. The Candy Man, the murderer who used doctored Halloween candy to hide killing his own kid, committed his crime in 1974. As a testimony to the power of folklore, by the time I hit my late prepubescence (the early 80's), this isolated incident had taken over people's perception of the holiday. By the time I was twelve, Halloween had become what it is now (more or less): a day for parties, aimed at both children and adults.
Sure, there're still some trick-or-treaters out there, but the holiday does seem to have lost its non-commercial joie de vivre (maybe, I'm just getting old). Many movie buffs credit the 1947 film Meet Me In St. Louis as crystallizing the ideal mid-America old-fashioned Halloween. In the movie, the costumed children meet around a bonfire (in the street!); one of them runs off and pulls a prank (throwing flour at a neighbor man).
My childhood Halloweens didn't involve bonfires or pranks, but I realized, watching the movie, that the costumed children are completely unchaperoned; that freedom was part of my childhood (my childhood was basically Sandlot). Whenever I get trick-or-treaters now, they are almost always accompanied by adults. To be honest, I probably would accompany my kid too if I were a parent. But when I was young, Halloween really was a kids' night--our parents simply expected us to be home before . . . midnight, I guess. My parents weren't careless; they were products of the Depression-era. Since my siblings and I are all still alive, I guess that sense of security (or was it a sense of "if it isn't poverty or a bomb, it won't kill you"?) was justified.
And the truth is, there have been relatively few incidents of children being harmed on Halloween. The Proquest Newspaper search I used to locate the "Candy Man" turned up almost no other incidents of poisoning. I think I read somewhere that more kids are hurt from carelessly crossing the street while trick-or-treating than from "stranger-danger," but that could be propaganda by those companies that sell glow-sticks.
Factually incorrect or not, the surge of poison/razor blade scares had great impact. However, human beings are in general great adapters. Halloween didn't go away; it morphed. After the scares of the 1970's and 80's, at-home types of entertainment became more and more popular: for example, decorating one's house or yard for Halloween. To be technical, these "haunted" or "spook" houses are folk performances rather than folk lore, but with holidays, the two tend to merge: folk performances inform the way the holidays are written and talked about.
Literary/Modern Examples: Children's literature contains some of the best writing about Halloween, namely The Witch Family by Eleanor Estes and Jennifer, Hecate, Macbeth, William McKinley, and Me, Elizabeth by E.L. Konisburg (Konisburg references the folk performance of the costume parade at school). There is also a reference to Halloween pranking in Ghosts Have I Been by Richard Peck.
For media examples, there are the Monk and Psych episodes that deal with poison candy (the Psych episode just makes a reference, but the Monk episode uses the idea of poison candy as its central mystery).
Also, Home Improvement has several Halloween episodes in which Tim creates a "spook" house. This is part of Tim's repairman-persona; there are also several Christmas episodes where he competes with a neighbor man--not Wilson--on how many lights and elves and electronic reindeer he can put up. Although this seems to be entirely commercial, these types of displays are in fact largely folk-promoted. I could digress here into a discussion of the things people put in their yards, but it would get way off-topic. I mean . . . gnomes?