"Hafele and Keating are credited with an experimental measurement which
confirms time dilation and matches predictions with an accuracy of about
10%. It gives an experimental answer to the twin paradox".
http://en.wikipedia.org/wiki/Hafele–Keating_experiment
http://hyperphysics.phy-astr.gsu.edu/HBASE/Relativ/airtim.html
This the first I've heard of that.