Here are a few to get things started:
Don Henley’s The Boys of Summer as an 80s synth tune
Bossa nova version of Crowded House’s Don’t Dream It’s Over
Reggae-ish version of Aretha Franklin’s Respect
A capella men’s ensemble version of Lady Gaga’s Bad Romance
EDIT: This has been great, everyone. Thanks to everyone for all the terrific links and for introducing me to a number of artists I wouldn’t have heard of otherwise. Seriously, the online rabbit-holes have been thoroughly enjoyable–even it I stayed up a bit too late a number of times.
Worth it.
Postmodern Jukebox ft. Haley Reinhart - Creep (original by Radiohead), jazz/big band style
Novena - Bury a Friend, progressive metal Billie Eilish. Starts out like the original and gets progressively heavier.
So many of the Postmodern Jukebox covers are fantastic.
Time, the Valuator covered Lovely as well. Pretty fucking amazing.