Virtual +Buy

"Virtual +Buy" is an informal term for cards that do not provide +Buy per se, but can serve a similar function to those that do in that they allow you to gain multiple cards per turn.

Cards that provide virtual +Buy include:

Black Market lets you buy a card during your Action phase which is completely independent from the normal process of buying cards.
Extra turn effects (for example Outpost, Possession, and Voyage) give you an extra turn which comes with a Buy like a normal turn.
Gainers and remodelers
Cards that genenerate multiple gains from a single Buy. While many of these effects are very restricted, often gaining a specific card only, some are rather flexible.

List of cards

Cards in italics have been removed.

Cards that are gained with additional cards

Border Village — a card cheaper that Border Village
Cache — two Coppers
Duchess — a Duchess
Farmland — a non-Farmland card costing exactly more than the trashed card
Noble Brigand — all Silvers and Golds trashed due to its Attack
Death Cart — two Ruins
Ferryman — a card from the extra card pile selected at setup
Masterpiece — a Silver per overpaid
Stonemason — two cards costing exactly the amount overpaid
Messenger — a card costing up to
Port — another Port
Crumbling Castle — a Silver
Fortune — a Gold per Gladiator in play
Haunted Castle — a Gold
Rocks — a Silver
Sprawling Castle — a Duchy or three Estates
Skulk — a Gold
Experiment — another Experiment
Camel Train — a Gold (Exiled instead of gained)
Hostelry — a Horse per discarded Treasure
Cursed — a Loot and a Curse
Rich — a Silver

Cards that provide gains when other cards are gained

Hoard — a Gold
Talisman — copies of all bought non-Victory cards costing up to
Haggler — a non-Victory card cheaper than the gained card
Duplicate — a copy of a gained card
Livery — a Horse
Architects' Guild — a non-Victory card cheaper than the gained card
Cutthroat — a Loot
Fawning — a Fawning card
Bureaucracy — a Copper
Growth — a card cheaper than the gained Treasure

Allowing cards to be bought without using Buy

Black Market

Events that gain multiple cards

Ball — two cards each costing up to
Banquet — two Coppers and a card costing up to
Pilgrimage — three differently named cards you have copies of in play
Raid — a Silver per Silver in play
Trade — up to two Silvers
Conquest — two Silvers
Demand — a Horse and a card costing up to
Invasion — a Duchy, an Action, and a Loot
Prosper — a Loot plus any amount of differently named Treasures
Gather — three cards, one each costing exactly , , and
Receive Tribute — three differencly named Action cards you don't have copies of in play
Windfall — three Golds
Alliance — a Province, a Duchy, an Estate, a Gold, a Silver, and a Copper
Commerce — a Gold per card gained the same turn
Enclave — a Gold and a Duchy (the Duchy is Exiled instead of gained)
Populate — one card from each Action Supply pile
Stampede — five Horses

Gallery

For a gallery of extra turn effects, see here.

Gained with additional cards

Gain with other cards