Cityscape. (Flash game) Build a city with 18 available components. The order in which those components are employed determines how they'll interact and how far the city will develop. There's one correct order that will score the full amount of points. (via)
Babble - a seriously addictive hybrid of Boggle and Scrabble.
The Mystery of Time and Space! [note: Flash] Rather involving game, if puzzlers like Myst are your bag, baby.
It's Friday, and you don't want to work. You want to practice your hacking skillz instead.
The "Hitchhiker's Guide to the Galaxy" Infocom game online! (Java-based) I think I just died and went to heaven. I played this years ago on a crusty old Mac - this and Fool's Errand. What old games do you remember spending hours or even days playing?
Addictive game based in large part on discussion. Perfect for MeFi'ers in my book. There's trivia, there's politics, plenty of "strategery", and the heart of it is the message boards. Plus the added benefit of very few trolls, because to make it in this game, you need to be intelligent and literate. No wonder I'm addicted.