The Myriad Planes of Existance
Oh, and while this is not directly related to the above - languages spoken.
Now, obviously you have your Common, Dwarven, Elven, Halfling, Gnome, and Orc - but there are many other languages, some of which will be familiar, and others not so much.
For example:
Goblin
Giant
Draconic
Auran
Celestial
Infernal
Druidic
Anticommon - If two people say the exact same thing at the same time in Common and Anticommon, both of them are jinxed and mute until someone says their name three times.
Quaintly Antiquated Common - Spoken by The Dead Sea Trolls.
Spanish - Spoken by Pinatas and Tequila Mockingbirds.
Japanese - Spoken by Cat Girls and Plutonium Dragons.
Dutch - The universal language of love.
French - The universal language of lust.
Blarney - The universal language of Blarney.
Strine - From down under.
L33T - Spoken by Net Trolls and Geeks.
Technobabble - Spoken by engineers.
Crazy Moon-Talk - Spoken by Moon Maidens.
Shnork - Spoken by Noses and Snot Elementals.
Venusian - Spoken by Amazons.
Martian - Spoken by Chauvinist Pigs.
Legelese - Spoken by Lawyers and Insurance Salesmen.
Reindeer - Spoken by Reindeer, right on the tin, there.
Squeak - The language of rats and similar beings.
Spoon - Spoken by Giant Cockroaches.
Harpo - Spoken by Harpies.
Kamen - Spoken by really ancient cultures.
Cornpone - Spoken by Resident Weevils.
Evil - ...it's Evil.
Squeeble - Spoken by the invincible diamond faeries, which don't even exist in this campaign.