I suppose most people’s top ten is based on what they like the most. I tend to think of sites as informative or enjoyable. I started playing with Second Life™ to follow friends leaving Myst Online, it closed but has since re-opened. So, I started out looking for informative and explanatory information about my avatar’s new home.
I still like knowing what is going on. I got hooked when mesh was in development. I had been building a huge building in OSGrid and realized mesh would let me do so much more. Now the time I spend building is mostly out-world (not logged in) rather than in-world. Where SL Groups used to keep me informed the user groups, formerly Office Hours, give me more meat about coming changes.