This is the one that everyone's talking about! Google has just launched Lively.
Lively lets you participate in and create 3D worlds from your browser (you just need to download a plugin) - and then use your Google account to log in and create your own avatar.
You can interact with other users in rooms just like you would do in Second Life. You can also watch YouTube clips and share your own photos.
Find out how to get started:
Then try out the Google room below.