Not Always Right is one I peruse daily.

I agree with the comment that many seem to involve customers who are mentally ill or otherwise impaired, and those can be a little sad. But others are pretty amusing. I think the best ones are the ones where a customer, in the middle of an abusive rant, realizes that he is completely in the wrong, and has made a huge scene. These usually end with "and runs out of the store".

I also recommend passiveaggressivenotes.com