I'm actually studying Old English now; they didn't give a rat's ass about "proper" usage in the ways we think of it (no standardized spelling, spaces between words, lines of poetry, etc.). It's also pretty exciting to wait til quite near the end of a sentence to find out what the verb is!
In terms of Modern English, however, my own pet peeve is the use of an apostrophe when the writer means to make something plural. It makes me absolutely crazy, and really inspires me to take the sort of guerilla punctuation action advocated by Lynne Truss in "Eats, Shoots and Leaves," to wit: carrying a large black sharpie at all times to correct such abominations as "Ice cream sundae's sold here." Ice cream sundae's WHAT sold here? And WHO is Ice cream sundae, anyway, and why aren't all three of his or her names capitalized?
I'm ever more convinced that the "Zero Tolerance Approach to Punctuation" is long overdue!