I've read quite a few blogposts about folks using their 2nd edition AD&D Forgotten Realms material as a setting for the new 5th edition Dungeons and Dragons. It makes sense to me given how much old Realms material I have around here. Many of the folks point to the 1st edition of the Forgotten Realms campaign setting and others advise using the second edition specific setting. Amongst my favorite resources are the Volo Guides.
The Volo guides are like a Fodor’s travel guide for the Forgotten Realms there are five Volo guides to various sections of Toril and one guide dealing “With all things Magical”. The guidebooks are packed full of real places that adventurers frequent in games- taverns, inns, brothels, shops, and places of interest (like haunted keeps and such). Just about every entry has some kind of hook ripe for development into an adventure. Before you know it your characters will have a list of favorite places to stay and shops to frequent in between dungeon crawls and tomb robbing.
Volo's guides are also a wonderful way to plan out down time for characters. for instance, if a character in your game needs a suit of plate armor, you can quote a price from the players book and be done with it or you could tell the player that the best plate armor in the sword coast can be found at a shop called The Halls of Hilmer on the street of bells in Waterdeep, thus a desire for plate armor leads the character on a journey to the most fabulous city in the Realms.
I find the information in them much more useful than stat-blocks for Uber-NPC's that I'll never use in my campaign These guidebooks have become the most referenced material I have on the Forgotten Realms. And speaking of stat-blocks, the guides have very little in the way of rules or stats, it's all about the flavor and feel of the establishment or locale, which makes it a smooth transition to 5th edition.
You can find almost all of them at RPGnow (it seems that the guides to the North and Baldur's Gate II have yet to be released).
Volo’s guide to Waterdeep
Volo’s guide to the Sword Coast
Volo’s guide to the Dalelands
Volo’s guide to Cormyr
Volo’s guide to all things Magical
