What makes a "Check" legal?

Working for a bank, I am learning a lot of the stranger sides of banking past and present. One of the more interesting factoids is that a check - technically a Negotiable Deposit Instrument - does not need to be on special paper produced by your bank. Rather, it can be any piece of paper with the critical info on it. The Straight Dope has all the info you need.

Charlie Rice, a columnist for the old This Week Sunday newspaper supplement, once wrote about various goofy checks that he claimed had been successfully cashed over the years:

Eben Grumpy of Iowa was a little slow in paying John Sputter $30 he owed him. Sputter threatened to sue, so Grumpy painted a check on a door and dropped it on him from a third-story window next time he came over. A court ruled the door was legal payment.

Albert Haddock of England paid his taxes by whitewashing a check for 26 pounds, 10 shillings on the side of a cow. The check was ruled legal.

A participant in an arc-welding contest in Cleveland won first prize for a steel check that he hand-lettered. The check was cashed by officials at a cooperative bank. "The canceling holes," Charlie says, "were applied by a bank guard with a submachine gun." Right.

One story from our local area was from a contractor who was being harrassed by a supply house with whom he had a dispute, because they had delivered substandard parts. After years, he lost the case in court and was hounded by a collection agency. In protest, he had the check finally chiselled into a cinderblock and sent it to the company C.O.D. so they had to pay postage, and then have the debate with the bank about cashing it. It was entirely legal.