I’m the sort of person who can’t avoid noticing spelling and grammar errors — They just shout out to me from the page/screen. I know most folks lack this curse and can simply read along, overlooking any errors — but not me.
So, I drop notes to webmasters (example) to alert them to errors, explain the corrections, and point out any broken links. Doing this redirects my annoyance, helps me stick to my principles (Be part of the solution.), and (I hope) improves the web.