I repress the urge to correct others' spelling on a daily basis because it seems that less people want to be corrected (and learn) than want to get offended by the fact that someone would correct them. I think people assume that someone correcting them is trying to take a position of superiority or something. It's a shame because there are people, particularly people whose first language isn't English, who do actually want to learn and would appreciate the corrections.

Firefox's built in spell-checker is very cool (though it has its flaws too).