What are the Four Immeasurables (aka Four Noble Abodes)? Also known as the Four Noble Dwellings or the Buddhist Virtues or the Brahmaviharas.  Where in the Pali Canon are these described?
According to http://en.wikipedia.org/wiki/Brahmavihara the Four Noble Abodes are:


Loving-kindness (Pāli: mettā, Sanskrit: maitrī) towards all: the hope that a person will be well; &quot;the wish that all sentient beings, without any exception, be happy.&quot;

Compassion (Pāli and Sanskrit: karuṇā): the hope that a person's sufferings will diminish; &quot;the wish for all sentient beings to be free from suffering.&quot;

Empathetic joy (Pāli and Sanskrit: mudita): joy in the accomplishments of a person—oneself or another; sympathetic joy; &quot;the wholesome attitude of rejoicing in the happiness and virtues of all sentient beings.&quot;

Equanimity (Pāli: upekkhā, Sanskrit: upekṣā): learning to accept loss and gain, good-repute and ill-repute, praise and censure, sorrow and happiness (Attha Loka Dhamma), all with detachment, equally, for oneself and for others. Equanimity is &quot;not to distinguish between friend, enemy or stranger, but regard every sentient being as equal. It is a clear-minded tranquil state of mind—not being overpowered by delusions, mental dullness or agitation.&quot;


According to the Metta Sutta, Shākyamuni Buddha held that cultivation of the four immeasurables has the power to cause the practitioner to be reborn into a Brahma realm (Pāli: Brahmaloka)

According to: http://www.accesstoinsight.org/lib/authors/nyanaponika/wheel006.html:

These four attitudes are said to be excellent or sublime because they are the right or ideal way of conduct towards living beings (sattesu samma patipatti). They provide, in fact, the answer to all situations arising from social contact. They are the great removers of tension, the great peace-makers in social conflict, and the great healers of wounds suffered in the struggle of existence. They level social barriers, build harmonious communities, awaken slumbering magnanimity long forgotten, revive joy and hope long abandoned, and promote human brotherhood against the forces of egotism.

They are found in the Pali Canon in Digha Nikaya 13:
Source: http://www.accesstoinsight.org/lib/authors/nyanaponika/wheel006.html

The Basic Passage on the Four Sublime States from the Discourses of the Buddha
I. Here, monks, a disciple dwells pervading one direction with his heart filled with loving-kindness, likewise the second, the third, and the fourth direction; so above, below and around; he dwells pervading the entire world everywhere and equally with his heart filled with loving-kindness, abundant, grown great, measureless, free from enmity and free from distress.
II. Here, monks, a disciple dwells pervading one direction with his heart filled with compassion, likewise the second, the third and the fourth direction; so above, below and around; he dwells pervading the entire world everywhere and equally with his heart filled with compassion, abundant, grown great, measureless, free from enmity and free from distress.
III. Here, monks, a disciple dwells pervading one direction with his heart filled with sympathetic joy, likewise the second, the third and the fourth direction; so above, below and around; he dwells pervading the entire world everywhere and equally with his heart filled with sympathetic joy, abundant, grown great, measureless, free from enmity and free from distress.
IV. Here, monks, a disciple dwells pervading one direction with his heart filled with equanimity, likewise the second, the third and the fourth direction; so above, below and around; he dwells pervading the entire world everywhere and equally with his heart filled with equanimity, abundant, grown great, measureless, free from enmity and free from distress.