Everything in Monopoly Online

Currently at v1.3.0 - the full list of what's built so far.

Accounts & Lobby

Free account creation and login
A live lobby: create or join a game (2-8 players), sequential game IDs
One active game per player at a time
Spectator mode - watch any game in progress, and see who else is watching it
Leave a game in progress at any point (counts as a loss; your properties return to the bank)
Last-activity tracking per game, shown in the lobby with an inactivity notice
Admin panel - manage donator status, delete stale games

Everything updates live for every player and spectator - no manual refreshing
A full game log, kept in full rather than truncated, newest entry first, with the latest highlighted
Properties shown grouped by color per owner, with owned/total counts for each group

Buy a property you land on, or decline (or can't afford it) and send it to a structured round-robin auction
Rolling doubles grants an extra turn; three in a row sends you straight to Jail
The full official 16-card Chance and Community Chest decks, with real shuffled draw and discard piles
Get Out of Jail Free cards - drawing one holds it in your inventory until you use it
Advance-to-nearest-railroad/utility cards, and Go Back 3 Spaces with chained landing effects (you can land on - and fully resolve - another Chance, Community Chest, or Tax square along the way)
General repairs / street repairs fee cards, charged per house and hotel you own
Double rent on a complete, unimproved color set
Even building and selling enforced across a color group, including four houses on every property in the set before any hotel
Sell houses and hotels back to the bank at half price
Mortgage and unmortgage properties - mortgaging is blocked while any property in that color group is improved

Propose a trade with any other player, at any time - even out of turn, or while in Jail
Trade cash, properties (mortgaged ones included), and Get Out of Jail Free cards
Accept, decline, or counter an offer - a counter replaces the same offer with roles swapped, rather than starting a new one
A complete, unmortgaged color set can be traded with its houses and hotels intact

Can't cover rent, tax, or a card fee outright? Mortgage properties or sell houses/hotels to try to raise the cash before going bankrupt
Give up and declare bankruptcy at any point instead of continuing to liquidate
Debt owed to another player hands that player everything you have - cash, every property (buildings carried over intact), and Get Out of Jail Free cards
Debt owed to the bank (tax, card fees) returns your properties to the bank instead