$20
1936
The last British Crown (5 Shillings) coin to be minted was the 1965 Churchill Commemorative.
The U.S. did not mint commemorative quarters in 2000, only the regular State Quarters series. These are only worth face value unless you have a proof or uncirculated one. If you're referring to something else, please post a new question with more details.
All 1988 Australian coins were minted at the Royal Australian Mint Canberra. Since they were all minted at the same place, there is no mintmark.
If you found it in change, 5 cents. Billions were minted.
There were 372,000,000 2004-D and 361,440,000 2004-P Peace Medal nickels minted. There were also 344,880,000 2004-D and 366,720,000 2004-P keelboat design nickels minted.
The first US wheat penny minted was the regular 1909.
If you refer to the British 1999 cupro-nickel Five Pound (Crown) coin "In Memory of Diana Princess of Wales" commemorative, there were over 5 million minted.
Not for general circulation as the metal is far more valuable the the face value of coins being minted, however silver may be added to very special commemorative coins.
The 2009 International Year of Astronomy commemorative 25 Euro coin minted in Austria. The related link has more details.
I think you coin may have been minted in the Marshall Islands and is worth about $15.
This medal, as well as others for other years from 1972 to 1976, were minted by the U.S. Mint For more details, see : http://www.workingmancoins.com/ForSale/BicMed.htm