The Million Names of God. -er, Tolkien.
Tolkien | J.R.R. Tolkien | John Ronald Reuel Tolkien | J.R.R.'s son, Christopher Tolkien

J.R.R.'s own "Middle Earth" work:
The Silmarillion, the creation and early history of Middle Earth.
The Hobbit, the story of Bilbo Baggins' discovery of the One Ring, the one thing that would turn the tide of Sauron's war on Creation.
The three books of The Lord of the Rings trilogy, building on the story of The Hobbit. As a good trilogy should be, there are too many intersecting stories to do it any justice here. Suffice it to say that LotR remains an influential, if not defining, ideal in the heroic fantasy genre.

Other peoples' work:
...So influential, in fact, that there are innumerable related products capitalizing on the captivating world of Middle Earth. Arguably the most profitable is Christopher Tolkien's voluminous exploration/interpolation of his father's notes. There is the obligatory satire, Bored of the Rings. Like any sci-fi or fantasy work worth its mindshare, there is also the obligatory role-playing game (MERP), and collector card game, intriguingly named "Middle Earth".

Middle Earth seems to appeal to pop musicians; recording artists from Led Zeppelin (Misty Mountain Hop, The Battle of Evermore) to Enya (Lothlorien) have captured their own unique visions of Tolkien's world. E2 user Infinity recommends Nightfall In Middle Earth by Blind Guardian. Could it be that artists resonate with Tolkien's own artistry, or just that Middle Earth and good pop music are both groovy when you're stoned? (By which I mean "petrified", as by the gaze of Medusa, of course.)

The Powers of Middle Earth:
Eru Iluvetar | Valar | Maiar | Melkor/Morgoth | Ungoliant | Glaurung | Balrog | Gandalf/Mithrandir | Saruman | Radagast | Sauron

Lothlorien on 20 silver pieces a day.
Angband | Valinor | Mirkwood | Rivendell | Moria | Mordor | Numenor | Rohan | The Shire | Umbar | Angmar | Gondolin | Arnor

The Creatures of Middle Earth:
If Middle Earth was important to fantasy fiction, it was positively seminal to role playing games. Elves, dwarves, orcs, halflings, dragons, ents, worgs, trolls, goblins and wraiths: we may owe the Norse myths for these archetypes, but we owe Tolkien for their popularity in fantasy art and literature. In keeping with the contribution of Arthurian hero stories, there are also plenty of regular humans, horses, ponies, dogs and wolves, and maybe a few extraordinary spiders and eagles.
Balrog | Mûmak | Beorn | Ent ...

Creative Linguistics:
Tolkien invented several languages for Middle-Earth; probably the best-known are the Elves' Sindarin and Quenya, but the Dwarves also used Khuzdul, and there are many others mentioned but not detailed in his works.

Tolkien Everything:
And of course, this node would be incomplete without some Everything Culture as seen through the eyes of an elf-hugger:
e2Tolkien (usergroup) | One Node to Rule Them All | The One Node | ...

More Middle Earth Miscellany:
A Elbereth Gilthoniel | Andúril | Celeborn | Galadriel | Dúnedain | Olvar | Kelvar | Gonnhirrim | Moriquendi | Cirdan | Tolkien's Languages | ...