While Googling some grammar questions the other day, I came across a goldmine: AskOxford's Ask the Experts page. (This is the US version -- there's also a UK version.) This page has links to tons of English language trivia, and it answered some nagging questions I had.
Here are some of my favorites:
There are so many answers on this site I'd love to link to, but I guess the list above is enough to get started. Go check out the rest (be sure to click on the category links, especially the Word Origins section).