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The expiration date on a medication only tells you how long the pharmaceutical company is willing to guarantee the effectiveness of a particular drug. It doesn't actually tell you anything about how long the drug is truly good for. Testing done by the US military has shown that about 90% of the drugs they have stockpiled are still good several years after their expiration date. Most drugs gradually break down with time, and some period of time they might be less effective, but still safe. The only drug known to actually become harmful after its expiry date is tetracycline. For drugs where getting the exact dose of the medication is really important (eg. oral contraceptives, heart medications, blood thinners, etc.) it's probably best to take them only before their expiry date. Also, some formulations like liquid antibiotics have pretty short shelf lives and shouldn't be taken after their expiry dates. For drugs like over-the-counter painkillers and the like, it's probably safe to take them after the expiration date.

References:
http://www.usaweekend.com/05_issues/050710/050710healthsmart.html
http://www.medscape.com/viewarticle/460159
http://www.mercola.com/2000/apr/2/drug_expiration.htm