What are your favorite quotes related to the topic metta?
One of my favorites is the ending to The Simile Of The Saw
I hope to add others.
The Simile of the Saw
“In the same way, there are these five ways in which others might criticize you. Their speech may be timely or untimely, true or false, gentle or harsh, beneficial or harmful, from a heart of love or from secret hate. When others criticize you, they may do so in any of these ways. If that happens, you should train like this: ‘Our minds will remain unaffected. We will blurt out no bad words. We will remain full of compassion, with a heart of love and no secret hate. We will meditate spreading a heart of love to that person. And with them as a basis, we will meditate spreading a heart like a catskin bag to everyone in the world—abundant, expansive, limitless, free of enmity and ill will.’ That’s how you should train.
Even if low-down bandits were to sever you limb from limb, anyone who had a malevolent thought on that account would not be following my instructions. If that happens, you should train like this: ‘Our minds will remain unaffected. We will blurt out no bad words. We will remain full of compassion, with a heart of love and no secret hate. We will meditate spreading a heart of love to that person. And with them as a basis, we will meditate spreading a heart full of love to everyone in the world—abundant, expansive, limitless, free of enmity and ill will.’ That’s how you should train.
If you frequently reflect on this advice—the simile of the saw—do you see any criticism, large or small, that you could not endure?”
“So, mendicants, you should frequently reflect on this advice, the simile of the saw. This will be for your lasting welfare and happiness.”
That is what the Buddha said. Satisfied, the mendicants were happy with what the Buddha said.
Another favorite is of course from the Metta Sutta which I chant nearly every day usually before I collect breakfast. Things start to get warm for me usually at this line here where I am reminded what not to do. Then comes the wonderful part of above below and all around…ahhh…
The Discourse on Loving-kindness Meditation
Na paro paraṁ nikubbetha, nātimaññetha katthaci na’ kañci,
No one should cheat another, nor should he despise anyone wherever he is,
byārosanā paṭighasaññā nāññam-aññassa dukkham-iccheyya.
° he should not long for suffering for another because of anger or resentment. 
Mātā yathā niyaṁ puttam-āyusā ekaputtam-anurakkhe,
° In the same way as a mother would protect her child, her only child, with her life,
evam-pi sabbabhūtesu mānasaṁ bhāvaye aparimāṇaṁ.
so too towards all beings one should develop the measureless thought (of loving-kindness). 
Mettañ-ca sabbalokasmi’ mānasaṁ bhāvaye aparimāṇaṁ,
Towards the entire world he should develop the measureless thought of loving-kindness,
uddhaṁ adho ca tiriyañ-ca, asambādhaṁ averam-asapattaṁ.
above, below, and across (the middle), without barriers, hate, or enmity. 
Tiṭṭhaṁ caraṁ nisinno va, sayāno vā yāvatassa vitamiddho,
Standing, walking, sitting, lying, for as long as he is without torpor,
etaṁ satiṁ adhiṭṭheyya, brahmam-etaṁ vihāram-idha-m-āhu.
he should be resolved on this mindfulness, for this, they say here, is the (true) spiritual life. 
Paja sabba sussayantu
Dusentu duggatim gamim
May all beings residing in their respective dwellings sleep soundly and dream pleasure dreams!
Being blessed in glory, may they wake early in the morning with auspiciousness!
May they be able to abstain from evil deeds, which lead to the four woeful abodes.
May they be able to fulfill the thirty perfections incessantly and attain spiritual maturity stage by stage!
Taking into consideration the situation of the present day, we will find that the (Bharamacariya) moral virtues, namely, loving-kindness (mettā), compassion (karuna) and sympathetic joy (Mudita) appear to have ceased to flourish, to have dried up in the human mind. The fire (element) generated by beings through such cessation of virtues incinerates even the virtuous, who thus find living a good moral life difficult.
From Abhidhamma in Daily Life, by Ashin Janakanbhivamsa, page 12
Also found here
When I see a monk who is late for breakfast and looks like he just woke up, I often jokingly tell him, “Ahhh… your metta is very powerful! Sukhaṁ supati ”
The quote of the first of the eleven benefits of metta is taken from Mettānisaṁsasuttaṁ
“Mettāya bhikkhave cetovimuttiyā,
“Monks, from the freedom of mind by friendliness meditation,
āsevitāya, bhāvitāya, bahulīkatāya, yānīkatāya,
when practised, developed, made much of, carried on,
vatthukatāya, anuṭṭhitāya, paricitāya, susamāraddhāya,
established, maintained, augmented, and properly instigated,
eleven advantages are to be expected.
What are the eleven?
He sleeps happily,
"Dwells pervading (intent upon) one direction with his heart endued with loving-kindness, likewise the second direction, likewise the third direction, likewise the fourth direction, and so above, below , and around; everywhere and equally he dwells pervading the entire world with his heart endued with loving-kindness, abundant, exalted, measureless, free from enmity, and free from affliction”
(Vibh 272; D I 250).
An antidote to anger
visuddhimagga. ix 38
he should try resolution into elements. How? “Now, you who have gone forth into homelessness, when you are angry with him, what is it you are angry with? Is it head hairs you are angry with? Or body hairs? Or nails? … Or is it urine you are angry with? Or alternatively, is it the earth element in the head hairs, etc., you are angry with? Or the water element? Or the fire element? Or is it the air element you are angry with? Or among the five aggregates or the twelve bases or the eighteen elements with respect to which this venerable one is called by such and such a name, which then, is it the materiality aggregate you are angry with? Or the feeling aggregate, the perception aggregate, the formations aggregate, the consciousness aggregate you are angry with? Or is it the eye base you are angry with? Or the visible-object base you are angry with? … Or the mind base you are angry with? Or the mental-object base you are angry with? Or is it the eye element you are angry with? Or the visible-object element? Or the eye-consciousness element? … Or the mind element? Or the mental-object element? Or the mind-consciousness element you are angry with?” For when he tries the resolution into elements, his anger finds no foothold, like a mustard seed on the point of an awl or a painting on the air.
The Metta Sutta (English) - YouTube This is the Metta Sutta chanted by the Monks of Abhayagiri Monastery in California
The Metta Sutta read by Thanissaro Bhikkhu (english) - YouTube Metta Sutta in english translation read by Thanissaro Bhikku
Ven. Balacitta (1) - Metta Bhavana - YouTube Ven. Balacitta (1) - surprising results of cultivating metta bhavana by spreading good will to unfriendly persons