Anno Online is the free-to-play online, browser-based version of the popular Anno game series. This game is developed and maintained by Blue Byte Software, and published by Ubisoft. This game does not require a 3D graphics card or any installation on the computer, so you can play it at home, at work or even at school on any computer that has internet access.
The gameplay itself is basically a medieval city-building simulator with online elements put into it. After registering and starting your colony, you will begin with a few small cottages, a marketplace, some lumberjacks and goat farms. Your goal is to amass resources, put your workers on different tasks, construct buildings and expand your colony to a powerful empire.
The game has standard online features, with chat, friends lists, guilds and alliances. You can exchange resources with other players via the markets. The game has an 'energy' meter, which is a bit limiting. This will prevent you from getting resources too quickly in the later parts of the game. This often prevents you from playing longer than a few minutes per session, because there isn't too much to do at any given time.
Graphics are surprisingly good for a 2D browser-based game, and the interface is pretty neatly organized and easy to use.