Elemental Evil - Princes of the Apocalypse story arc. It is a free download, and is available either from the WotC website or through DriveThruRPG.
The whole thing is 25 pages long, and consists essentially of two sections; races and spells.
The races include aarakocra (bird-men who first appeared in the 1E Fiend Folio), Deep Gnomes (aka Svirfneblin, underdark-dwelling gnomes who first appeared in the 1E adventure D2 Shrine of the Kuo-Toa), Genasi (descendants of trysts between mortals and genies, with attendant magical powers, first found in the 2E Planewalker's Handbook), and goliaths (stone-like beings related to giants, who first appeared in the 3.5 Races of Stone).
The aarakocra are now native to the elemental plane of air, and aside from the powers you'd expect based on their physical form (they're able to fly, and can use their talons in combat), they don't seem to be particularly over-powered.
Deep gnomes were originally quite over-powered, but in this version they seem to be quite well aligned with other player races (there is an optional feat that allows a deep gnome character some innate magical power, but even then it's nothing compared to the 1E ability to summon earth elementals).
Genasi come in four types (air, earth, fire, water) and each has some magical powers and damage resistance appropriate to their respective genie heritage. I'd be hard pressed to see why a player might choose not to play one, given that some of them are quite handy.
Finally, goliaths are physical powerhouses, getting bonuses of 2 to STR and 1 to CON, and some other size and toughness induced skills as well. While perhaps not quite so overpowering as a half-giant from Dark Sun or a half-ogre from Greyhawk, I can see some players making this their default race when creating a "tank".
While I like the fact that the 5E team is obviously self-consciously plucking material from across the history of D&D, and it's nice to see more free material, I found myself disappointed that there was nothing class-based herein. No backgrounds, no new clerical domains (indeed, nothing new for clerics at all!), no new arcane traditions, no new druid circles, no new sorcerous origins, and no new warlock patrons. The only new feat is associated with the deep gnome race. It's entirely possible that the Companion will be updated to include this sort of material, and I do feel somewhat guilty complaining that a free supplement doesn't have more stuff in it, but it does seem that something billed as a Player's Companion would have some more of those basic building-blocks of character construction.
As for my own purposes, I will certainly be able to use some of the new spells in my Temple of Elemental Evil project. The deep gnome race will naturally come in handy if I ever do anything underdark-related. But I don't really see allowing the other character races as PC choices, except perhaps for a very specific short-term game. Still, it's worth getting, as the price is right and any DM running a 5E game will certainly find the spells at least useful.