We can refer here to the music of bagpipes (wind sounds), Ravi Shankar (string sounds), and the historical works of La Monte Young (‘Trio for Strings’) who is one of Eno's references.
-- Sweelinck
La Monte Young is one of the true masters of drone. His work spans decades, and includes work with John Cale (in Young's "Theatre of Eternal Music" ensemble) who would later bring Young's ideas into the Velvet Underground. It's also worth taking several hours to check his work "The Well-Tuned Piano", which involves a specially retuned grand piano on which Young would do a lot of sustained repetition that eventually blurs into a drone. He and his partner Marianne Zazeela (who also works on light projection design for Young's works) even lived in their "Dream House", a combination of a residence and sound installation, with the drone's basis pitch coming from an amplified fishtank pump.
Another great piano "dronemeister" would have to be Charlemagne Palestine, who's also done works for electronics and pipe organ. Palestine's "strumming" method is so hard on the instrument that broken piano strings are a fairly common occurrence at his concerts. And Mazz already mentioned the beautiful and austere works of Eliane Radigue, in which everything constantly evolves toward and away from drone textures.
Also, there's a number of "Krautrock" artists who were/are superb drone creators. In that column, I'd have to mention Klaus Schulze's initial works ("Irrlicht" and "Cyborg"), Tangerine Dream's "Zeit", early Popol Vuh ("Affenstunde" and "In den Garten Pharos") and the dark and foreboding first album by Gunter Schickert, "Samtvogel". Even Kraftwerk's first couple of albums have some heavy drone elements along with really alien soundscaping.