The Impact of the Hubble Space Telescope

I'm in the throes of writing a muckity-muck review article on the scientific impact of the Hubble Space Telescope. I've got plenty of opinions about what aspects of HST were transformative in my own subfields, but I thought I'd throw open the floor to the CV readers as well. What I'm looking for are key results that (1) truly changed how people thought about something, and (2) would (probably) not have been discovered without HST. Bonus points if you include an actual paper reference, and/or you work on stars or planets!