I know the OP is looking for suplement recommendations, but for me some of the most inspiring material has been the novels set in the FR. There's the famous
Legend of Drizzt series, featuring the drow Drizzt Do'Urden, which begins in the FR's Underdark in the novel Homeland and IIRC continues on for 16 more novels. It seems like just about everyone and their dog has read those - well maybe just those owner's of a dog with an Intelligence 10+. But there's less known novels series which I've found to be well worth the read. Here's a few from my list;
The Yellow Silk It's my favorite D&D novel. It's part of a series called the Rouges and is set in the city of Spaneyliyon within the coastal kingdom of Altumbel. For me it's most memorable for having one of the -if not the- nastiest Bards I've ever read about. The hero is a Shou-Human, which you don't read about often in FR books.
The Black Bouquet It's also part of the Rogues and some consider it the best in the series - I'm sticking with my Yellow Silk. Its set In the city of Oeble in the Border Kingdoms and feature a lot of interesting plot twists. It of course features a Rogue-thief, who ends up with much more than what he bargained for.
The Last Mythal Trilogy A series featuring very cool, ancient artifacts called Mythals and far ranging among Faerun, beginning west in the Moon Shae islands and as far east as the Dalelands. It features some big, memorable battles. I particularly liked the lore around the Mythals, which I inserted in one of my homebrewed adventures.
Those are the most memorable for me. I've read a good deal more novels set in FR, but those are the series that come to mind.