Here are the D&D gods as presented in 4th Edition, mapped to similar gods from existing religions. I tried to stick to relatively well-known pantheons, which made it difficult to get close fits for most of the evil gods.
All the gods' descriptions here are of course very over-simplified. If you want to know more about the gods mentioned, I suggest GodChecker and Wikipedia.
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Good
Avandra - Hermes
Old: Greek trickster god of traveling and boundaries
New: Goddess of change, luck, trade and travel
Bahamut - Themis
Old: Greek goddess of justice
New: God of justice, honor, nobility, and protection
Moradin - Thor
Old: Norse fighter-god with a big hammer
New: God of creation, artisans and loyalty
Pelor - Ra
Old: Egyptian god-king of the sun, fertility, and just about everything else
New: God of the sun, summer, agriculture and time
Unaligned
Ioun - Taliesin
Old: Celtic bard hero-god who ate the Salmon of Knowledge
New: Godess of knowledge, prophecy and skill
Corellon - Apollo
Old: Greek god of learning, the arts, archery, and healing
New: God of arcane magic, spring, beauty and the arts
Erathis - Athena
Old: Greek godess of civilization, the organized aspects of war, and wisdom
New: Goddess of civilization, invention and laws
Raven Queen - Hecate
Old: Greek witch-godess of oaths, magic, and the underworld
New: Goddess of death, fate and winter
Kord - Zeus
Old: Greek god-king of storms and patron of many warriors
New: God of storms, strength, and battle
Melora - Artemis
Old: Greek goddess of the wilderness and slaughter
New: Goddess of the wilderness and the sea
Sehanine - Dionysus
Old: Greek trickster-god of reversals, wine, and arriving at places
New: Goddess of trickery, the moon, love, and autumn
Evil
Asmodeus - Balor
Old: Celtic god-demon who ruled over armies of demons
New: God of power, domination and tyranny
Bane - Set
Old: Egyptian evil god of warriors and armies
New: God of organized battle, war, and conquest
Gruumsh - Ares
Old: Greek god of bloodlust and the battle cry
New: God of turmoil and destruction
Lolth - Eris
Old: Greek goddess of chaos and crashing parties
New: Goddess of spiders, shadows and lies
Tharizdun - Ahriman
Old: Zoroastrian source of all evil
New: Secret, imprisoned god of evil and death
Tiamat - Tiamat
Old: Babylonian creator water goddess who spawned hordes of monsters
New: Goddess of wealth, greed, and vengeance
Torog - Tartaros
Old: Greek personification of Tartaros, the lowest level of Hades
New: God of underdark, imprisonment and pain
Vecna - Kuk
Old: Egyptian god/goddess of darkness and secrets
New: God of undeath and secrets
Zehir - Apophis
Old: Egyptian snake god of evil and poison
New: God of darkness, poison and serpents