I always greet a single magpie and enquire how his wife is. It's not really a genuine superstition for me (i.e. I don't actually believe in it); it's just a silly little joke that I have with myself. (Similarly, I've always tried to avoid looking at a full moon through glass, although other ABers have since told me that it's meant to be a new moon that I shouldn't be looking at anyway!)
Superstitions clearly bother some people though. I've supervised surveys carried out by the developers of Great Western Park in Didcot. It's a massive development, about the size of a small town, where there will eventually be 3,300 homes. However not a single one of them will be numbered '13'. All of the odd-numbered houses on each street have Number 11 followed by Number 15 and flats are similarly numbered. The developers clearly think that, even in these 'enlightened days', many prospective buyers would shun a house or flat numbered '13'.