Mu

Death Note has this to say about Mu: "When they die, the place they go is Mu (nothingness)..."

"They" can be interpreted as either:

A. Humans in general

-or-

B. Humans who have used a Death Note

Humans who have used a Death Note and gone to Mu
Light Yagami Misa Amane Jack Neylon Teru Mikami

Humans who have used a Death Note and will go to Mu
(People who haven't died in the canon series) Near

Speculation about Mu
In many fanfictions, Mu is depicted as the Shinigami Realm. However, this is not canon, and is mostly for entertainment value.

The term "Mu," as it is used in Death Note, may have originated from the name given to an Atlantis-like lost continent written about by James Churchward in 1926's "The Lost Continent of Mu, Motherland of Man." -Mogturmen 22:52, 17 September 2008 (UTC)-