WebGL, the 3D technology that's associated with HTML5, continues to make giant strides in diverse areas:
Exploration of human anatomy: Zygote Body, released yesterday, and BioDigital Human, the successors to Google Body (previously)
World Visualisation: WebGL Earth, Nokia's 3D Map of the entire earth (previously). WorldWeather and The WebGL Globe, a Google project that displays all kinds of data. Also: Where Does My Tweet Go?
Games: browser ports of Team Fortess 2, Quake 3 and Rage (a developer’s diary). SkidRacer, an entire game in WebGL. Mini Mass Effect (not yet playable, sadly).
Tools: 3Notes.js, a visual scene editor. Developer documentation. More resources.
Play Trivia to Help Save Abandoned Animals. Free Rice set the standard for on-line charity games and Free Poverty soon followed by donating water. Now you can play for free kibble to be donated to either Rocket Dog Rescue or The Urban Cat Project.
Geosense: an online world geography game. Play alone or with others using a guest account or your own unique ID.
Place the State. How well do you know the fly over states?
NationStates.net allows you to create your own country, decide how it starts out by answering a short questionary, and then it gives you issues to solve (one per day, though you can set it to two per day). How you answer those issues determines how your country fares, the type of country it is, and many other things. You can join the UN and elect a regional representative, create your own region that you and your friends can migrate to (all new nations start out in The Pacific, which is consequentially the largest region in NationStates, but you can move wherever you want).