I recently used the West Virginia statehood stamp on an envelope to Susan.
And a stamp from 2016 commemorates the 50th anniversary of Star Trek's debut on television. This is one of a set of four stamps on an envelope to Carroll.
Here's a stamp on an envelope I received from Katerina in Greece. It is one of five that celebrates the 150th anniversary of the National Archaeological Museum of Athens, and features the god Poseidon.
Katerina also included this piece of mail art inside her envelope.
For links to more commemorative stamps, head over to Sunday Stamps.