Maybe you have some of these in your personal collection, or perhaps there are some at your bank, just waiting to be purchased at face value!
1964 Kennedy half dollar
At 90% silver, the value of this coin is closely pegged to the value of silver. As of April 1st, 2011, the value of this coin is ~$13.68 – a good return on the 50 cents face value! Also keep on the look out for the 1965-1970 Kennedy half dollars they are 40 cents and today worth ~$5.59. The condition of the coin is rarely important, a very nice coin (uncirculated) may get you a slightly better price, but a badly worn coin will still have the silver value, so it should sell for the silver price.
When silver prices started skyrocketing, all the dimes and quarters from before 1965 quickly disappeared from circulating currency as collectors grabbed them and the US Mint removed them as well. However, nickels had silver in them for a few years during World War II. The 1943-1945 nickels were 35% silver, and many 1942 ones were as well. ~$2.12 is what one of these would net you, more than forty times the face value.
Silver values constantly fluctuate, so make sure to check them out before you make a deal. The best site for silver coin values is coinflation.com, with real time silver values.