Screw you. Pay me. 
Oh the horror stories I've heard:- a software house whose biggest client found out that they were 75% of their business.  So they just stopped paying their bills and waited them out until the source code ended up in receivership.- A contractor that bid a time and material job, deployed it successfully, and got paid.  He was then called back in to change a few things for which he was not paid because, "We paid for it to be right in the first place."- a local trucker that made 500 successful deliveries buy screwed one up, so he got paid nothing.  He blocked the parking lot with his truck in order to get paid.  (The company would rather pay him that get the bad press from the local newspaper.)- bounced checks, checks without signature, checks where the 2 amounts didn't match, checks with next year's date: too many to mentionAlso, I'm surprised no one has mentioned a controversial practice I know that others have used: time bombs in the software.  It goes something like this:  Here is your working version which we will make permanent when you're paid in full.I have never resorted to something like this and I hope I never do.  Has anyone ever tried this?  What are the ramifications, financial, business, and legal?