Who vs. Whom. Lots of resources on the web clarify when to use what pronoun, but I'm more interested in justifications for or against having this distinction in the English language. Use is weakening, and I'm all in favor of evolutionary language and ditching old concepts that are falling out of use. But it's good to understand the reasons why it was ever a rule in the first place.

Ask a Linguist is very old (five years) and very cool. No on-site search, but a site-limited Google search takes care of that.