Although I've already played through Uru, I got into the closed beta for Uru Live. It's quite amazing to explore with other people in an MMO type feeling. It's completely different to Riven, but it gives such good history on the D'ni and all that.

The worlds in Uru are amazing to explore. The character models are bit plastic, but the worlds are the best design and graphics I've seen in any MMO to this date.