Video Games are a huge part of our culture today, especially our student population. Games are more engaing, exciting, and real then ever before. They give the user the ability to be emersed into a world where they can socialize with other players all over the globe, coordinating attacks and working together to solve challenges. In many ways video games can be used to foster educational goals and objectives. Perhaps we may not bring a Wii or X-box into the classroom anytime soon (although they would be awesome on a large projector screen), but there are similar web based games that can act as great ways to engage students.
Here are some great web based game resources. There are thousands out there for this topic so please share, wiki away and contribute your favs!

NASA has once again landed on the lunar surface with the goal of colonization, research, and further exploration. Shortly after the return to the Moon, NASA has established a small outpost on the south pole of the moon called Moonbase Alpha. This is an interactive game (free) that allows the user to experience what it may be like to live and operate in space.
Whyville is a virtual world geared for teen and pre-teen girls and boys. Inside Whyville, citizens learn about art history, science, journalism, civics, economics, and really so, so much more. Whyville works directly with the Getty, NASA, the School Nutrition Association, and Woods Hole Oceanographic Institution (to name just a few) to bring incredible educational content to kids in an incredibly engaging manner.