Keter

Keter (, "keṯer" "Crown") also known as Kether, is the topmost of the sephirot of the Tree of Life in Kabbalah. Since its meaning is "crown", it is interpreted as both the "topmost" of the Sephirot and the "regal crown" of the Sephirot. It is between Chokhmah and Binah (with Chokhmah on the right and Binah in the left) and it sits above Tiferet. It is usually given three paths, to Chokhmah, Tiferet and Binah.
Keter is so sublime, it is called in the Zohar "the most hidden of all hidden things", and is completely incomprehensible to man. It is also described as absolute compassion, and Moses ben Jacob Cordovero describes it as the source of the 13 Supernal Attributes of Mercy.
Keter is invisible and colorless.
Description.
According to the book Bahir:
"What are the ten utterances? The first is supreme crown, blessed be His name and His people." 
The first Sephirah is called the Crown, since a crown is worn above the head. The Crown therefore refers to things that are above the mind's abilities of comprehension. All of the other Sephirot are likened to the body which starts with the head and winds its way down into action. But the crown of a king lies above the head and connects the concept of "monarchy", which is abstract and intangible, with the tangible and concrete head of the king.
This first Sefirah represents the primal stirrings of intent in the Ein Soph (infinity), or the arousal of desire to come forth into the varied life of being. But in this sense, although it contains all the potential for content, it contains no content itself, and is therefore called 'Nothing', 'The Hidden Light', 'The air that cannot be grasped'. Being desire to bring the world into being, Keter is absolute compassion.
The name of God associated with Keter is Ehyeh Asher Ehyeh (Hebrew: אהיה אשר אהיה), the name through which he revealed himself to Moses from the burning bush. "It is from the name Ehyeh that all kinds of sustenance emanate, coming from the source, which is the infinite".
Keter, although being the highest Sephirah of its world, receives from the Sephirah of Malkuth of the domain above it (see Sephirot). The uppermost Keter sits below no other Sephirah, although it is below Or Ein Soph which is the source of all Sephirot.
Qualities.
Ethical behaviour.
Moses ben Jacob Cordovero, in "The Palm Tree of Devorah", discusses ethical behaviour that man should follow, related to the qualities of the Sephirot, in order that man might emulate his Creator. Humility is the first, because although Keter is the highest, it is ashamed to look at its cause, and instead gazes at those below it. One's thoughts should be pure, one's forehead should display no harshness, one's ears should always turn to hear good, one's eyes should distance themselves from noticing evil, always looking at the good, one's nose should be free from the breath of anger, one's face should always shine, and his mouth should express nothing except good.
13 Supernal Attributes of Mercy.
Through discussion of a line in the Michah, 13 attributes are associated with the Sephirah Kether:
Who is God like you, who pardons iniquity and forgives the transgressions of the remnant of his heritage? He does not maintain His anger forever, for He delights in kindness. He will again show us compassion, He will vanquish our iniquities, and You will cast all their sins into the depths of the sea. Show faithfulness to Ya'akov, kindness to Avraham, which You have sworn to our fathers from days of old. ("Michah", 7:18-20)
Accordingly, the 13 attributes are derived from this and are described in great detail.
Additionally, the "Thirteen Attributes of Mercy" were described by Rabbi Chizkiyah in an allegorical depiction of a lily among thorns. The metaphor in whole is known and taught as "The lily amongst the thorns," a phrase found in Shir Hashirim 2:2. A summary:
The secret of spiritual protection is revealed through a richly metaphorical discourse given by Rabbi Chizkiyah. The Rabbi explains that the spiritual forces that protect and watch over us are called the 13 Attributes of Mercy. They are transmitted into our physical world through the first 13 words of the Torah. When judgments are decreed against us, these 13 forces can safeguard us from their influence. We begin drawing this Light of protection to ourselves at the very moment we begin to browse and behold the mystical shapes and sequences of the Aramaic text, and to learn the spiritual insights presented there. 
Non-Jewish practices.
In "The Mystical Qabalah" Dion Fortune describes Keter as pure consciousness, beyond all categories, timeless, a point that crystallises out of the Ein Soph, and commences the process of emanation that ends in Malkuth.
The name of God given to it is Eheieh, the archangel that presides over it is Metatron, the order of angels that resides in it are the Holy Living Creatures (the Chaioth ha Qadesh, Hebrew: חיות הקדש), and its mundane chakra is said to be the First Swirlings of the cosmos (Primum Mobile, Rashith ha Gilgalin).
A.E. Waite made mention that Rabbi Azariel ben Menachem, a student of Isaac the Blind, in his "Commentary on the Sephiroth" granted a particular colour to each Sephira, yet these do not agree with the colours given in the Zohar, where Kether (which, according to him, is also correlated to "Yechidah") is called colourless, Tiphareth purple, and Malkuth sapphire-blue.
As pure formless consciousness, it is often compared with the Sahasrara chakra, which resides above the crown of the head, in Indian Shakta Tantra.
In Aleister Crowley's "Liber 777", Keter is associated with the Four Aces of the Tarot deck, White Brilliance, Poseidon, Brahma, Wodan, Zeus, The Trinity, Almond in flower, Diamond, Elixir Vitae, Dao, and Death (not a complete list of the 777 associations).
It is said to have a negative aspect, the Qliphothic order of demons called Thaumiel ruled by the Archdemons Satan and Moloch.
Keter is also identified with the planets Neptune or Pluto, the Atma in Theosophy and Raja Yoga, and the "Khabs am Pekht" in Egyptian mysticism.
In popular culture.
In the SCP Foundation mythos and related games, "Keter" is one of four major classifications for anomalous objects or entities (Safe, Euclid, Keter, Thaumiel). Specifically, "Keter" classification denotes entities or objects that are exceedingly dangerous and difficult to contain consistently or reliably, with containment procedures often being extensive and complex.
In "Tales of the Abyss", the "Sephiroth" are described as physical pillars keeping the world from sinking into a poisonous mud. Each major town has a Sephiroth supporting it, and one such town is Keterburg, the northernmost town.
Swiss metal band Samael have released a song titled "From Malkuth to Kether" on their "Exodus" EP.
In both the anime "Fate/Apocrypha" and the mobile game "Fate/Grand Order", the Servant Solomon ibn Gabirol/Avicebron possesses a Noble Phantasm named Golem Keter Malkuth.
In David Bowie's "Station to Station", he refers to "one magical movement from Kether to Malkuth".