In many works, there is a God of Evil. An ultimate embodiment of evil that seeks to either spread unhappiness or kill as much as he/she wants.
This is the inverse of that, a deity that seeks to defeat evil and/or seeks to spread goodness. In most works with a God of Good, he/she will often (but not always) be the Big Good, guiding The Hero but not getting directly involved in his/her quest to defeat the Big Bad. In some cases, the god of good's role can be spreading good will or actively repairing that which has been tainted by evil or simply maintaining the world from plain old decay. It might seek to convert all to the setting's standard of goodness, warping hate into love, despair into hope, obsession into indifference where appropriate.
There is a tendency for Gods of Good to be female rather than male. Also, Beauty Equals Goodness (and by proxy Hot God) tends to come into play with this trope.
In many works, there is a conflict between the God of Good and the God of Evil. Often, as a take-off of Satan, the God of Evil is a rebellious underling of the God of Good.
Not in any way to be confused with God Is Good, which is when there is a single god who is good (though in most monotheistic faiths, Godis given this title, and many others like it, so He actually still counts).
To avoid Flame Bait and Edit Wars, note that this trope isn't about gods who simply are good, it's about gods of goodness. Gods who are nice but mostly defined by other qualities do not qualify. If the god can, by the setting's definition, do evil without contradicting their very being, they don't count.
The Pony POV Series has Celestia and Luna's parents, Fauna Luster and the unnamed Father of Alicorns, both of whom appear to be Anthropomorphic Personifications of existence. In addition, the Father appears to be the Dimension Lord of, and may very well be, Pony Heaven (at the very least, it's contained in His body).
Religion and Mythology
In Christianity, Islam, Judaism and others, God is often referred to as God of Mercy, God of Love, God of Justice etc. And yes, sometimes God of Good or Goodness. Of course, he's the God of pretty much everything, but these are perhaps his most frequently invoked titles.
In Islam, the invocation of God (a short phrase used at the start of just about anything—every single prayer, journeys, work, meals), bismi-llāhi r-raḥmāni r-raḥīm, means "In the name of God, most Merciful and Kind", and is quite possibly the single most common phrase any muslim will utter, regardless of language or nationality.
In the Bible God admits to being God of evil (2 Kings 6:33, Isaiah 45:7, Lamentations 3:38, arguably the Genesis story). Specifically he is responsible for evil's existence but does not accept this as an excuse for people to be wicked. God hates evil deeds and will punish those who commit them but still loves all people and does not take pleasure in the death of evil doers (Jeremiah 31, Lamentations 3:32, Ezekiel 18:23, 18:32). In Hosea God does declare he is going to stop loving sinners and hate them but later admits he cannot do it, still loves them and cannot bring himself to punish them as they really deserve.
The buddhist goddess Guanyin, who is the deity of compassion and loves all unconditionally.
Ra in Egyptian Mythology. While his role was mostly as the sun god, he was pretty much considered the pinacle of morality and "goodness", opposed to Apophis. Ma'at more so even: she was the personification of moral decency.
A domain is literal or metaphysical field the god has complete mastery of, a God of Fire would have a domain of Fire, etc.
And in 3rd edition, the norm is that every deity who is good in alignment is enough of a God of Good that he or she will grant access to the Good Domain; similarly with all of the gods who are evil in alignment
4E's core pantheon has four of them - Bahamut, Pelor, Avandra and Moradin.
3E's core pantheon had Heironeous, Moradin again, Yondalla, Pelor again, Garl Glittergold, Ehlonna, Corellon Larethian and Kord.
From Dragonlance: Branchala, Habbakuk, Kiri-Jolith, Majere, Mishakal, Paladine, Solinari
From the Forgotten Realms (and there's quite a list of them!): Chauntea, Lathander, the late Mystra, Selune, Sune, Torm, the late Tyr, Bahamut again, Mielikki, Tymora, Deneir, Eldath, Lliir, Lurue, Milil, and a host of others in other lands of Toril.
Magic: The Gathering has Avacyn, the goddess who gives hope and protection to the humans of Innistrad against the demons, vampires, and other assorted horrors that inhabit it. She was, in fact, specifically created by the vampire Planeswalker Sorin Markov to serve as the Good in the Balance Between Good and Evil of the plane since the humans were being driven to extinction by everything else, which is a bad thing in the long run since that will lead to the self-destruction of Innistrad.
BIONICLE Mata Nui is the Great Spirit and creator of the Matoran Universe, he created the Matoran, the Rahi and even the Makuta.
The Goddess Palutena of Kid Icarus, the liege of Pit.
Double Subverted in Fable. The first game has Avo, whose temple the player can make donations to for beneficial gifts. The updated rerelease reveals that "Avo", however, is just a huge mass of Will energy set up by a merchant to scam people. That's the subversion. The "double" part kicks in during game two, in which the Temple of the Light in Oakvale seems to be more or less the same... except the being they worship turns out to be more than just Will energy: it's sentient and can even speak to you depending on your choice of quests. This seems to imply the existence of a higher, benevolent power; even if it isn't Avo.
To some extent, Princess Celestia from My Little Pony: Friendship Is Magic. While her most explicit role is being the goddess of the sun, one of her roles as stated by the Mayor of Ponyville is being the "Bringer of Harmony".
Transformers Cybertron: Primus, Lord of Light and Creator-God of the Transformers, is usually in this role whenever he decides to make an appearance in person. Most of his time is spent in altmode (the planet Cybertron itself).