Although Greyhawk, Dragonlance, and many other campaign settings have rather robust pantheons of original deities, the Forgotten Realms pantheon (even ignoring culture-specific regions such as Mulhorand for now) is an interesting mixed bag including several gods from historical pantheons, as well as many original deities as well.
From the Celtic mythos, we have the greater gods Silvanus and Oghma; from the Finnish mythos we have the goddesses Mielikki, Loviatar, and Ilmater (the last who apparently had a sex change and is now a god); finally, from the Norse mythos we have the one-handed god Tyr. Less obviously, there is also the goddess Tyche from the Greek pantheon, who was split into the goddesses of Tymora and Beshaba, as well as the goddess Bast of the Egyptian pantheon combined with Felidae and Zandilar the Dancer to become Sharess; finally, there is of course Tiamat, whose Realms variant borrows simultaneously from the Babylonian pantheon and the original AD&D five-headed dragon.
Besides historical sources, the Forgotten Realms pantheon has also borrowed heavily from (and expanded upon) the "standard" Greyhawk pantheon, including the four elemental lords of Akadi, Grumbar, Istishia, and Kossuth, but also nearly the entirety of the demihuman pantheons, with some modifications - some of the more notable additions include the drow goddess of good Eilistraee, the psionic duergar goddess Deep Duerra, and the triumvirate elven goddess of Angharradh (who is the Realms "combined" form of Sehanine Moonbow, Aerdrie Faenya, and Hanali Celanil). The Realms-only jungle dwarves have been given their own god as well (Thard Harr), but other races have been left deity-less (the arctic dwarves). Nearly all monster deities have been mentioned in Forgotten Realms sources as well, although almost none have been given a full treatment.
Although the Realms pantheon is in many ways a strange amalgam of different sources, it is very well-detailed with long, official writeups on the various churches, priesthoods, and rituals of each god. The specialty priest classes are in particular very attractive (and probably over-powered, but that is another story), making the various priest classes a lot more fun to play.