I have sent this Mercury Project stamp in the past, featuring Alan Shepard, the first American in space:
It was issued in 2011 alongside this stamp about the Messenger Mission, the first spacecraft to orbit Mercury:
Adrienne sent me this one - 1985 Canadians in space.
And I also received this one featuring the space shuttle: one of a set of 8 se-tenant stamps from 1981 on US space achievement.
Smash sent me this great 10c Skylab stamp issued in 1974.
The envelope she sent with that stamp was pretty special. I had to go hunting for the value for the 'B rate' stamp - 18c.
USPS Stamps has an interesting article about astronauts taking first day covers into space.
And finally you can go to Infinity and Beyond with Buzz Lightyear.
I have used his stamp on mail, but not received one myself.