There are a lot of things that need to be unlocked in DD.
For all unlocks that involve finding something in a dungeon run, you are required to leave the dungeon alive. You do not have to win, and the item will not be unlocked if you die.
This page is still under heavy construction, as are many of the pages it links to.
Human: Human Settlement. Starting race.
Elf: Elf Settlement. Found via quest progression.
Dwarf: Dwarf Settlement. Found via quest progression.
Halfling: Halfling Settlement. Found in a sub dungeon. Will join if given a health potion.
Gnome: Gnome Settlement. Found in a sub dungeon. Will join if given a mana potion.
Orc: Search Western Dungeons for sub dungeon with Orc who will have you drink a potion.
Goblin: Search Southern Dungeons for a sub dungeon with a Goblin who wants money (20 gold).
Fighter: Guild lvl 1
Berserker: Guild lvl 2
Warlord: Guild lvl 3
Wizard: Mage Tower lvl 1
Sorcerer: Mage Tower lvl 2
Bloodmage: Mage Tower lvl 3
Priest: Church lvl 1
Monk: Church lvl 2
Paladin: Church lvl 3
Thief: Thieves' Den lvl 1
Rogue: Thieves' Den lvl 2
Assassin: Thieves' Den lvl 3
Tinker: Found in sub dungeon enclosed by walls (Requires Alchemist unlocked)
Crusader: Found in a sub dungeon after unlocking all 9 Gods, requires 100 piety with any god
Transmuter: Found in a sub dungeon randomly. Unknown if any pre-req
Chemist: Found in a sub dungeon, requires destroying the walls for an item.
Half-Dragon: Requires completing Dragon Isles with 3 classes
Gorgon: Requires completing Naga City with 3 classes
Vampire: Requires completing Halls of Steel(Vicious) with 3 classes
Goatperson: Upgrade the Goat Glade added with the Goatperson DLC
Rat Monarch: Requires completing Demonic Library with 3 classes
Wraith: Unlock Berserker Class
Goo Blob: Unlock Warlord Class
Gorgon: Unlock Rogue Class
Bandit: Unlock Assassin Class
Serpent: Unlock Monk Class
Dragon Spawn: Unlock Paladin Class
Goat: Unlock Sorcerer Class
Golem: Unlock Bloodmage Class
Glowing Guardian: Found in the East (sub dungeon with path and 3 Zombies of increasing level)
Dracul: Found in the East (Sub dungeon with your clone, you are locked in) (prerequisite: Glowing Guardian)
Mystera Annur: Found in the North (Sub dungeon with 999 hp dragon, creative use of magic will get you past him)
Jehora Jeheyu: Found in the North (Sub dungeon has imps and an alter, you will need ENDISWAL to escape) (Requires Mystera)
Taurog: Found in the West (Sub dungeon.. large room with Taurog altar at the center)
Binlor: Found in the West (Sub dungeon small room surround in walls, smash walls to find alter)(Requires Taurog)
Tikki Tooki: Found in the South (Sub dungeon with reviving enemy, unlocked by killing all forms without taking a hit) (Requires The Earthmother)
Earthmother: Found in the South (Sub dungeon filled with plants)
Pactmaker: Found anywhere (Sub dungeon giant room with alter in the middle, touch alter with all other gods unlocked to unlock) (prerequisite: Unlock any god)
Hexx Ruins: Defeat Southern Swamp
Ick Swamp: Find Naga sub dungeon after completing Hexx Ruins at least once.
Naga City: Complete Ick Swamp
The Labyrinth: Defeat Western Jungle
Dragon Isles: Defeat The Labyrinth
Halls of Steel(Hard): Defeat Creeplight Ruins
Halls of Steel(Vicious): Defeat Halls of Steel(Hard) with 4 classes
Namtar's Lair: Defeat Halls of Steel(Hard)
Shifting Passages: Defeat Northern Desert
Demonic Library: Defeat Shifting Passages
Tower of Gaan-Telet(Hard): Opened by quest progression in game and getting one of every trophy.
Tower of Gaan-Telet(Vicious): Opened by completing Tower of Gaan-Telet(Hard)
Bazaar lvl 1: Buy Portal scroll and exit the level without dying.
Bazaar lvl 2: Quest Reward, buy all shops in a single run
Bank lvl 1: Complete Dangerous Investments and Venture Capital
Bank lvl 2: Complete
Witch lvl 1: Eastern Tundra sub dungeon. You have to gather 5 items from killing plants
Witch lvl 2: 600 gold
Alchemist lvl 1: Buy Philosopher's stone and exit the level without dying.
Alchemist lvl 2: 1200 gold
Alchemist lvl 3: 1200 gold
Explorer's Guild: City Expansion quest reward
Tavern: City Expansion quest reward
Taxidermist: Taxidermy quest reward
Human Settlement: Default
Guild lvl 1: Default
Guild lvl 2: 1,000 gold, requires Guild lvl 1
Guild lvl 3: 2,000 gold, requires Guild lvl 2
Mage Tower lvl 1: 150 gold, requires Taxidermy quest completion
Mage Tower lvl 2: 1,000 gold, requires Mage Tower lvl 1
Mage Tower lvl 3: 2,000 gold, requires Mage Tower lvl 2
Church lvl 1: 150 gold, requires Taxidermy quest completion
Church lvl 2: 1,000 gold, requires Church lvl 1
Church lvl 3: 2,000 gold, requires Church lvl 2
Thieves' Den lvl 1: 150 gold, requires Taxidermy quest completion
Thieves' Den lvl 2: 1,000 gold, requires Thieves' Den lvl 1
Thieves' Den lvl 3: 2,000 gold, requires Thieves' Den lvl 2
Blacksmith lvl 1: 200 gold, requires Thieves' Den lvl 1, Mage Tower lvl 1, or Church lvl 1
Blacksmith lvl 2: 600 gold
Blacksmith lvl 3: 1250 gold
Elven Camp: City Expansion quest reward
Elven Settlement: 200 gold, requires Elven Camp
Dwarven Camp: City Expansion quest reward
Dwarven Settlement: 200 gold, requires Dwarven Camp
Gnome Camp: Find the Gnomes quest reward
Gnome Settlement: 500 gold, requires Gnome Camp
Halfling Camp: Find the Halflings quest reward
Halfling Settlement: 500 gold, requires Halfling Camp
Goat Glade: 3000 Gold, requires Special Edition (or Goatperson DLC) content, and having unlocked 4 Altars.
Shop Scroll: Bazaar lvl 2
Apothecary: Bazaar lvl 2
Black Market: Thieves' Den lvl 2
Bet on the Boss: Thieves' Den lvl 2
Smuggler Den: Thieves' Den lvl 3
Patches the Teddy: Thieves' Den lvl 3
3-Slot Locker: Guild lvl 2
6-Slot Locker: Guild lvl 3
7 8 & 9-Slot Locker: Purchased from lvl 3 Bank individually
Magnet Fire: Mage Tower lvl 2
Extra Glyphs: Mage Tower lvl 2
Fewer Glyphs: Mage Tower lvl 2
Extra Boosters: Mage Tower lvl 3
Guaranteed God: Church lvl 2
Extra Altar: Church lvl 3 (requires all 9 gods)
Expanded Potion Kit: Witch
Sword: Blacksmith lvl 1
Shield: Blacksmith lvl 1
Bear Mace: Blacksmith lvl 2
Perseverance Badge: Blacksmith lvl 2
Slayer's Wand: Blacksmith lvl 2
Really Big Sword: Blacksmith lvl 3
Compression Seal: Alchemist lvl 1
Transmutation Seal: Alchemist lvl 1
Translocation Seal: Alchemist lvl 2
Vicious Token: Oh, Horatio! quest
Whurrgarbl: Berserker Silver
Trisword: Warlord Silver
Item: Midas Gloves Midas Gloves: Rogue Silver
Item: Agnostic's Collar Agnostic's Collar: Paladin Silver
Blue Bead: Sorcerer Silver
Item: Venom Blade Venom Blade: Assassin Silver
Vampiric Blade: Bloodmage Silver
Martyr Wraps: Monk Silver
Witchalok Pendant: Halflings Ho! quest
Fire Heart: Ice and Fury quest
Mage Plate: Wizard Silver
Battlemage Ring: Minecraft quest
Hero's Helm: Confidence quest
Crystal Ball: From completing quest to win Den of Danger with second tier classes.
Soul Orb: Hello, gnomes! puzzle pack
Viper Ward: Hello, halflings! puzzle pack
Dragon Soul: Bonus Bravery: Oasis quest
Rock Heart: Bonus Bravery: Slime Pit quest
The Steam version of Desktop Dungeons has Achievements. These do not unlock anything, and are not present in other versions of Desktop Dungeons.