Very few things are as offensive to me as badly punctuated or misspelled signs. It's one thing to misspell words and misuse commas, apostrophes, and such in your own writing. It's another thing entire to assault the world with a sign that reads, "Cake's for sale everyday." There oughta be a law!
As such, back when I lived in Texas and the Great Ann Richards was governor, I wrote her a letter arguing that the state needed a department to regulate all public signage: the Texas Department of Grammar. Agents would drive around the state and inspect any and all signage placed where the public could see it; infractions (from misspelling to bad punctuation, etc.) would result in citations and fines. Great way to generate income for the old state guvmint! If a signage owner got three citations in a year, then he'd lose his right to post signage for one year. I argued that we had to do something to combat all the ignorance and bad grammar for which Texas is famous!
Needless to say, I got a nice form letter from Ann, but no job. (After Ann left, GW ran up the state's first-ever deficit; imagine if I'd been around to generate dollars!) Still, now that the blogosphere is here, people everywhere are forming their own online departments of grammar! It's so great to see that someone else shares my pet peeves for correct grammar and punctuation on signage.
Check out some of these links:
Apostrophe Abuse -- a gold mine of apostrophe errors
Curious Signs -- badly written signs of all kinds
The "Blog" of "Unnecessary" Quotation Marks -- a hilarious look at people's obsession with putting things in quotation marks
Apostrophism -- more apostrophe errors