The Wikipedia Game
Game type: browser-based, live against other players.
Time demand: Each individual round only lasts two and a half minutes, so it's not very time-demanding... unless you get sucked in.
What it is: This is just a cute little game that's fun to play now and again. The concept and execution are simple: you are given a starting wikipedia page, a page to end up on, and the idea is to click on links in the starting page (and following pages) until you manage to wind up on the target page.
For instance, on a game I just played, the start was Diablo III, and the end page was Coffee. The path I took went through "World of Warcraft" to "Internet Cafe" to "SF Net Coffee House" to "Coffeehouse" to "coffee," for six pages altogether (counting the start). There are multiple people all playing at once, listed to the left in a chat window, and the size of their paths (along with if they are one of the winners of the round, ie those that complete the path and how many steps it took) are displayed next to their username. (With a long guest name for those without an account/not logged in.
And... that's pretty much it, really. Simple, but great for a bit of fun now and again, and an exercising of the brain muscles. Recommended!