Du'as for Ramadan

When beginning the fast - Suhur

وَبِصَوْمِ غَدٍ نَّوَيْتَ مِنْ شَهْرِ رَمَضَانَ

Wa bisawmi ghadinn nawaiytu min shahri ramadan

"I intend to keep the fast for tomorrow in the month of Ramadan."
[Abu Dawud]

When breaking the fast - Iftar

اللَّهُمَّ اِنِّى لَكَ صُمْتُ وَبِكَ امنْتُ [وَعَلَيْكَ تَوَكَّلْتُ] وَعَلَى رِزْقِكَ اَفْطَرْتُ

Allahumma inni laka sumtu wa bika aamantu [wa 'alayka tawakkaltu] wa 'ala rizq-ika aftarthu

"O Allah! I fasted for You and I believe in You [and I put my trust in You] and I break my fast with Your sustenance"
["wa 'alayka tawakkaltu" is quoted in some books of knowledge - but not all, hence it is in brackets]
[Abu Dawud]

ذَهَبَ الظَّمَأُ وَ ابْتَلَّتِ الْعُرُوقُ، وَ ثَبَتَ الأجْرُ إنْ شَاءَ اللَّهُ

dhahabadh-dhama'u wab-tallatil 'urūūqi, wa thabatal arju inshaAllah

"The thirst is gone, the veins are moistened and the reward is confirmed, if Allah [Ta'ala] Wills"
[Abu Dawud 2:306]

اللَّهُمَّ إِنِّي أَسأَلُكَ بِرَحْمَتِكَ الَّتِي وَسِعَتْ كُلَّ شَيْءٍ أَنْ تَغْفِرَ لِي

Allaahumma inni as'aluka birahmatika al-lati wasi'at kulli shay'in an taghfira li

"O Allah, I ask You by Your mercy which envelopes all things, that You forgive me."

[A du'a that Abdullah ibn Amar (radiAllahu anhu) used to say when breaking his fast - as reported by Ibn Abi Mulaykah (radiAllahu anhu)]

When someone offers you food when you are fasting and you decline

"When you are invited to eat, then reply to the invitation. if you are fasting, then invoke Allah's blessings (on your host), and if you are not fasting then eat."[Muslim; 2:1054]

When you are fasting, and someone is rude to you

اِنَّيْ صَائِمٌ ، اِنِّيْ صَائمٌ

Inni sa'iimu, inni sa'iimu

"I am fasting, I am fasting"
[Sahih al-Bukhari, Fath al-Bari of Al-Asqalani; 4:1-3, Muslim; 2:806]

Upon Sighting of the new Moon

اللَّهُ اَكْبَرُ ، اَللَّهُمَّ اَهِلَّهُ عَلَيْنَا بِالآَمْنِ وَلاِيمَانِ ، وَالسَّلَامَةِ وَالاِسْلَامِ ، وَالتَّوْفِيقِ لِمَا تُحِبُّ رَبَّنَا وَتَرْضَى ، رَبُّنَا وَرَبُّكَ اللَّهُ

Allahu Akbar. Allahumma ahillahu 'alayna bil-amni wal-imaani, was-salaamati, wal-Islami, wat-tawfeeqi lima tuhibbu Rabbana wa tardha. Rabbuna wa RabbukAllahu

"Allah is the Greatest. O Allah bring us the new moon with security and faith, with peace and in Islam, and in harmony with what our Lord Loves and what pleases Him. Our Lord and your Lord is Allah"
[at-Tirmidhi 5:504, ad-Darimi 1:336]

Recited at intervals of taraweeh prayers

سُبْحَانَ ذِى الْمُلْكِ وَ الْمَلَكُوْتِ سُبْحَانَ ذِى الْعِذَّْْةِ وَ الْعظْمَةِ وَ، الْهَيْبَةِ وَ الْقُدُرَةِ ، وَالْكِبْرِيَآءِ وَ الْجَبْرُتِ، سُبْحَانَ الْمَلِكِ الْحَىِّ الَّذِىْ لايُنَامُ وَ لا يَمُوُتُ، سُبُّوحٌ ، قُدُّوْسٌ، رَبُّ الْمَلَئِكَةِ وَ الرُّوُحِ لَآ اِلهَ اِلَّا اللَّهُ نَسْتَغْفِرُ اللَّهُ ، نَسْئَلُكَ الْجَنَّةَّ وَ نَعُوُذُ بِكَ مِنَ النّارِ

Subhana dhil Mulki wal Malakuti, Subhana dhil izzati wal aDhmati wal haybati wal Qudrati, wal kibriyaa'i wal jabaroot Subhanal Malikil hayyil ladhi, la yunaamu wa la yamūtu, Subbuhun, Quddusun, Rabbul malaa'ikati war-rooh
La ilaha illal lahu, nustugfirullahi
nas 'alukul jannati, wa na udhubika min an-naar

"Glorified is the Owner of the Kingdom of the earth and the heavens; Glorified is the Possessor of Honour and Magnificence and Awe, and Power and Greatness and Omnipotence
Glorified is the Sovereign, the Living, Who does neither sleep nor die
O all Glorious, All Holy one, Our lord and the Lord of the Angels, And the soul.
There is no God but You, Forgive us, Grant us Paradise, and save us from (hell) fire."

Upon breaking the fast in someone's home

أفْطَرَ عِنْدَكُمُ الصَّائِمُونَ، وَ أَكَلَ طَعَامَكُمُ الأبْرَارُ، وَ صَلَّتْ عَلَيْكُم ُ الْمَلائِكَةُ

Aftara 'indakumus saa'imuna, wa akala ta'aamakumul-abraaru, wasallat 'alaikumul mala'ikat

"May those who are fasting break their fast in your home, and may the dutiful and pious eat your food and may the angels send prayers upon you."
[Abu Dawud 3:367, ibn Majah 1:556, an Nasa'i]

On Lailatul Qadr - the Night of Power

Aishah (radhiya Allahu Ta'ala anha), that she said: "O Messenger of Allah! What if I knew which night Lailatul-Qadr was, then what should I say in it?" He said 'Say

اَللَّهُمَّ اِنَّكَ عَفُوٌّ ، تُحِبُّ الْعَفْوَ فَاعْفُ عَنِّي
Allahumma innaka 'affuwwun tuhibbul 'afwa fa'fu 'anni' "

"O Allah You are The One Who pardons greatly, and loves to pardon, so pardon me."
[at Tirmidhi]