One of the best sites in the world is called http://www.roblox.com/. It is a virtual building game where you build buildings, enemies, power-ups, and weapons to make one big 3-dimensional world. You can make goals in it, so it can be a game, or you can just make it a place to roam. You can also dress up your character/avatar in different types of clothing. Just so you know, You have to install it. Also, people with Mozilla Firefox have to start it from the Windows Start Menu.
P.S. You have to buy the clothing with Robux and Tickets. They are like money that you earn from playing and showing up each day.