In 2007, Peter Davison returned in the Children in Need short "Time Crash" alongside David Tennant, and most recently in 2013's 50th anniversary special episode, "The Day of the Doctor", David Tennant's Tenth Doctor appeared alongside Matt Smith as the Eleventh Doctor and John Hurt as the War Doctor, as well as brief footage from all of the previous actors.
What was the name of the 50th Anniversary episode?
The Day of the Doctor