For US coins, 90% silver coinages ceased in 1964, and clad coinage started in 1965. However, in half dollars, 1965 - 1970 were 40% silver, and didn't start being 0% silver until 1971.
Nickels contained silver from mid-1942 - 1945. On these, the mintmark was moved to above the dome of Monticello, and for the first time, the Philadelphia mint was marked "P".
The only special coin I can think of off-hand for the clad coinage is the 1970-D half dollar, which was sold in mint sets only (I think), so you aren't likely to dig one up.
I hope this helps.
-- Tom