Actually oral agreements are very enforceable. The problem is proving the intent of the parties when there is a breach. When there is no written contract to determine the intent of the parties to the transaction, the court has to look to the conduct of the parties to determine their intent. The difficulty arises in the evidence presented to enforce the claim (i.e. the credibility of the witnesses, the written correspondence making demand for payment, the possible admissions of fact contained in the defendant's rebuttal of the demand, or his acquiescence in the claim by failing to respond to the demand, etc.)

There are also implied contracts which are very enforceable in which not a single word is spoken between the parties. These are contracts implied in fact and contracts implied in law. The most common example of a contract implied in fact is the purchase of groceries. When you bring your shopping cart to the checkout counter, your conduct in placing each item on the counter is an implied offer to buy the items even though there is no verbal or written offer. The check out clerk's conduct ringing up each item is an implied acceptance of the offer. Your conduct in paying for the items is both the consideration for the contract and your performance of your contractual obligations. The clerk's conduct in bagging the groceries and releasing them to you is his consideration for the contract and his performance of his obligations.

There is also an equitable remedy called a contract implied in law (also known as quasi contract, quantum meruit and unjust enrichment). It is not a true contract, but rather an equitable enforcement of an obligation by the court to the extent necessary to avoid injustice. If the defendant has accepted goods or services from you under circumstances in which he knew or should have known that you expected to be paid, you would be entitled to the recovery of the reasonable value of your goods or services. The problem is in proving their reasonable value. This is a remedy commonly used by landscapers when they plant shrubs on the wrong property. I had a case several years ago in which an oil company delivered oil to the wrong address.

There is also something called promissory estopple. If someone makes a promise to you, and you make a major change in your position to your detriment, the court will  enforce the promise made to you to the extent necessary to avoid injustice.

While I agree with Scott that it is always best to have your agreement in writing, the law is a living and ever evolving entity. It has many safety valves built into it to provide alternate theories of recovery and protection.

When I litigate a contract claim based on express written or oral contract, I always back it up with with an alternate claim for a contract implied in law. If the facts warrant, I will also include alternate claims for contract implied in fact and promissory estopple.