Even if you have a relatively small amount of assets, like $10,000 in cash, stock, any real property or other property of value a will can save a lot of problems. In almost all USA states and some countries, if one dies without a will, certain laws will automatically kick in to distribute monies in certain formulas as to a parent if a minor, to a spouse, parents and siblings or to others down the family per the law if there was no immediate beneficiaries after any debts and funeral costs are paid for. That would involve the supervision of a court (Surrogates Court in same USA states) to supervise the allocation. A will can help a lot with preventing persons who you don't want to get or shouldn't get your money. I would advise to get an attorney, 'do it yourself' wills can have serious flaws that can cause expensive complications. A will should also designate a replacement or successor beneficiary if one main beneficiary passes away as well as successor Executor or Co-Executors designated.
One should also draft, revise or redo their will if they have major changes in their lives or in the lives of their beneficiaries. For examples: if get engaged, get married or have a civil union with a same gender partner, or get divorced, have or adopt children, if a designated beneficiary dies or get married, gets government disability or certain income based benefits. Sometimes you just don't want your someone in the family to get the money if you die for personal reasons.
While you are getting your will done, don't forget to have prepared 2 other important documents. One is a 'Medical Directive', sometimes called a 'Living Will', to make sure your wishes are carried out as to if you do or do not want extraordinary measure take to keep you alive as well make medical decisions if your unable to do so. Second is a 'Power of Attorney' to allow someone (including an attorney) to take care of your financial business or make other important decisions if you become incapacitate temporarily or long term.
Also don't forget to designate and update as needed any designated beneficiaries of any employee 401(k) or other retirement benefits, beneficiaries of any life insurance as well as certain investments.
I would note that it wasn't until my father died in 2007 and me at age 52 to do all of the above or update 401(k) or life insurance beneficaries. Don't you wait so long.
[Edited 2010-01-18 17:55:25]
[Edited 2010-01-18 17:56:30]