One of my very favorite and free resources is Shortcuts by Jeff Harris.
He creates fun, colorful and informative activity pages on all sorts of topics like X-rays, Jellyfish and this week's Chess. These pages full of history, puzzles, comics, and jokes are published each week in lots of newspapers. You can read about the chracters and find links to downloadable teacher's guides at shortcutscomic.com. They also sell poster-size versions of these encyclopaedic comics. I like to cut these out and laminate them for students who finish early.
Comic image from comic.com