"Second Life" and "IMVU" are two virtual world games geared to older users. Both games require a download. For younger players, "Club Penguin" is an excellent virtual world that can be played directly in a browser. All three are free to play but offer in-game purchases to unlock more features.
In "IMVU" and "Second Life," players create a human avatar to represent themselves in the virtual world. The virtual games offer various locations for users to chat, including some user created areas. Both games allow users to design content that they can then sell themselves for either in-game currency or real money.
In "Club Penguin," users make a penguin to use as their avatar. As a Disney game, "Club Penguin" is heavily censored and ideal for younger players.