I finally caught one of the student essay sites, eCheat.com, red-handed.
Compare:

Mark Twain and American Anti-Imperialism, uploaded by
anoncollegestudent on Nov 29, 2004
http://www.echeat.com/essay.php?t=25783

with

An American Anti-Imperialist: Mark Twain on the Philippine-American
War, by Jim Zwick, published in Filipinas Magazine (Sept. 1992)
http://www.historyillustrated.com/ai/twain/amermtph.html

I have seen brief snippets from a number of my essays about Mark
Twain advertised at those sites for years but this is the first time one of
them has had the full text out from behind their subscription firewalls.

If any of your students have handed that in as their work, please let me
know what grade I got. :-)

Right after catching them red-handed, I also caught the State of New
Jersey red-handed:

HSPA Language Arts Tutorial Cluster 5: Reading Persuasive Text
http://www.state.nj.us/education/njpep/assessment/hspa/hspa_la/modul
e_5/reading_persuasive_b.htm

(sorry, you'll probably have to cut and paste that URL)

This is apparently widely distributed to teachers and students throughout
the state's school system, and is available for worldwide access at its
site. I also found it reprinted at an individual teacher's site at a New
Jersey school.

Does the State of New Jersey have some kind of special dispensation
that allows it to appropriate intellectual property, or should I be paid for
this like I would be if it was included in a textbook or course pack?

It has been depressing checking Google to see if my new site is showing
up there. Some pages do, but so far they list more copyright violations
than pages from my site.

Jim Zwick