If you found it in change it doesn't really have any collector's value so its only a nickel.
In uncirculated condition, as of 02/2015 it could retail for anywhere from a quarter in the lowest collectible grade (MS60) to several hundred dollars in top condition (MS66 or higher) so it may be worth having it evaluated by a dealer or appraiser.
Note that there are no mint marks on any US Coins dated 1965 through 1967.
August 19, 2009: If you got it in change spend it, the coins are not silver or rare. In 1966, 3 US mints were producing coins but none of them were putting mint marks on their coins. Therefore there is no way to know where a particular coin was made. This nickel can still be found in circulation and thus its circulated value is 5 cents. The uncirculated values are shown in the following list:
Uncirculated Grades........
MS60................................$0.20
MS63................................$0.35
MS64................................$1.80
MS65................................$15
MS66................................$125
MS67................................$1,060
25 cents, all circulation quarters from 1965 to date are made of copper-nickel,
10 cents.
10 cents unless in a mint set. It is in common circulation and made out of copper-nickel and contains no premium over face value.
1966 (US) dimes are not silver but are copper-nickel and commonly come up in circulation. Unless it is in a mint set, it is only worth 10 cents. Even uncirculated examples outside of mint sets carry little to no premium and little collector demand.
Unless it is uncirculated, it's pocket change.
5 cents
Australia has never produced a "nickel" coin.
It's the usual practice of this site to answer a single question at a time. Please see:"What is the value of a 1940 US nickel?""What is the value of a 1942 US nickel?""What is the value of a 1944 US nickel?"
50-500 usd
As of 16 Feb 2018, the melt value of one US nickel is 4.4 cents.
It's a very common coin that's still in found in circulation and only face value.
This design is called either a buffalo nickel or an Indian head nickel; both names are about equally common. Please see the question "What is the value of a 1919 US nickel?" for more information.
The US nickel was first issued in 1866. Please double-check your coin.
Well... it depends on who wrote the letter. For example, if I had written the letter in 1966.. it wouldn't be worth a plugged nickel!
5 cents.
A 1957 nickel is worth face value only, unless it's uncirculated or a proof coin.
In average condition, both coins are only worth their face value.