A few years ago, after getting screwed out of about $2000 by a client who got their site then changed the FTP password and stopped emailing me, I started putting a little insurance policy in every site I built. I wrote a little javascript that checks my server for a bit of data, and if I set that "killbit" on my server, the script would display a nicely designed pop up message over the client's site which basically says "the client didn't pay, but look how great the site looks, it was designed by me". I never had to use it, but always secretly wanted to. The best part is that it would be easily circumvented by any old programmer the client hired to get rid of it, but as soon as that new programmer saw what they were getting rid of, they'd probably drop the client themselves, or at least raise the price and demand payment up front.