BBC Proms
Henry Wood encouraged composers to write new music by performing it at the Proms.  Today this tradition continues as the BBC often commission composers to write new music for the Proms.  Some pieces which had their first performance at the Proms have become very well-known, e.g. The Confession of Isobel Gowdie by James MacMillan (first performed in 1990), or John Tavener's The Protecting Veil (first performed in 1989).