The Metta Suttas

The Buddha's Words on Kindness (Metta Sutta)

This is what should be done
By one who is skilled in goodness,
And who knows the path of peace:
Let them be able and upright,
Straightforward and gentle in speech.
Humble and not conceited,
Contented and easily satisfied.
Unburdened with duties and frugal in their ways.
Peaceful and calm, and wise and skillful,
Not proud and demanding in nature.
Let them not do the slightest thing
That the wise would later reprove.
Wishing: In gladness and in safety,
May all beings be at ease.
Whatever living beings there may be;
Whether they are weak or strong, omitting none,
The great or the mighty, medium, short or small,
The seen and the unseen,
Those living near and far away,
Those born and to-be-born,
May all beings be at ease!
Let none deceive another,
Or despise any being in any state.
Let none through anger or ill-will
Wish harm upon another.
Even as a mother protects with her life
Her child, her only child,
So with a boundless heart
Should one cherish all living beings:
Radiating kindness over the entire world
Spreading upwards to the skies,
And downwards to the depths;
Outwards and unbounded,
Freed from hatred and ill-will.
Whether standing or walking, seated or lying down
Free from drowsiness,
One should sustain this recollection.
This is said to be the sublime abiding.
By not holding to fixed views,
The pure-hearted one, having clarity of vision,
Being freed from all sense desires,
Is not born again into this world.

　
And here is a slightly different version read by Thanissaro Bhikhu:

Here is an essay by Thannissaro Bhikkhu about bringing wisdom to the brahmaviharas: 

And Finally! An overview from Wikipedia on brahmaviharas and metta.

– with thanks to Jude Vanderhoff for suggesting the Metta Suttas, providing the text and compiling the resources.