United States copyright law matters where I teach. (Most countries these days have somewhat similar laws about what the United States calls "fair use" and the somewhat different concept of what teachers are licensed by law to do for preparing classroom materials, but that can vary a bit from country to country.) Because United States law is reasonably generous to teachers,

http://www.copyright.gov/circs/circ21.pdf

http://www.teachingcopyright.org/curriculum/hs

I do make use of copyrighted materials to prepare class handouts (especially), but because there are restrictions of reproductions of materials even for teachers, I generally prefer to use materials that have been put on the Web specifically for use by other teachers. That's because then I know there is a license from the copyright holder for the use I intend.

There are at least two organizations now for sharing classroom materials, one a commercial service that charges fees,

http://www.teacherspayteachers.com/

and one coordinated as a nonprofit organization, for sharing free materials,

http://www.sharemylesson.com/

but when I visit those sites, I rarely find anything suitable for my classes, so I usually do direct searches by subject on Google, sometimes site-restricting to sites of educational institutions, to find suitable materials for my classes.