I've heard people say "for all intensive purposes" a few times. It's annoying.
And I've also seen "would of" written instead of "would have" and "should of" instead of "should have". In that case, I can understand it because it's the way it's usually pronounced, even though it's wrong. The first one is just people getting it wrong.
Something that might be worth noting is that we're having this discussion on an Internet message board, a medium that not everyone embraces. I don't have any stats to back it up, but if I had to guess, I would think that the average literacy rate (whatever that means) is somewhat higher here than the general population. We have chosen to communicate using a medium that promotes well-written statements and positions, as opposed to the quick-think-type format of a chat room, or text messaging. Again, I don't know, but I'd think that that means more of us would tend to know a bit more about grammar than average, because we use it more and choose to do so. Not necessarily causation, but correlation.