The Harp of Rassilon was a key to a secret door in the High Council chambers. A painting beside the harp showed a music score for a piece of music known as "Rassilon's Lament". (PROSE: Blood Harvest)
By playing the music shown in the painting on the harp, a secret chamber would open which contained the Time Scoop controls once used illicitly by Borusa. The Fifth Doctor discovered this function, leading him to uncover Borusa's misdoings. (TV: The Five Doctors)
When the Doctor returned to Gallifrey, he found that the harp had been replaced with a water feature. The Doctor jokingly asked if it was the water feature of Rassilon. (AUDIO: Time In Office)
The Eleven used the harp. (AUDIO: The Eleven)