"Uru" is a large, highly detailed real-time 3d fantasy/puzzle/adventure game based on Cyan Worlds' "Myst" series. (Check GOG.com for some great deals on earlier Myst games) - Uru contains a variety of beautifully designed, imaginative worlds which players can explore, as well as a rich, unique fantasy mythology and intelligent, creative, friendly fan community.

Uru has reopened...

After a long wait, "Myst Online: Uru Live" is open again, for free! 

Start playing Uru at MystOnline.com. <-The official site!
Here's a helpful video tutorial for getting started - it's from the previous version of the game but is still useful:
Myst Online: Uru Live 101

Yes, Uru really is free to download and play - though if you like the game and want it developed further, you can (optionally) donate a few dollars to the open-source Uru project, to help keep the servers running and fund production of new content for the game.

Myst and Uru-Related documents

Myst_history.pdf  Back by popular demand, my PDF summary of Myst/Uru storylines is available here.  This offers a condensed version of the Myst canon, including summaries of all novels and games in the series.

urustats.pdf <-analysis of player count and financial condition of Uru.  This document will cover growth of the game, based on data provided by Cyan Worlds.

Other links of note:
http://www.guildofwriters.com   Myst fans creating worlds for Uru.
http://www.guildofmessengers.com/  Myst/Uru related news source.
http://www.guildofgreeters.com/  Guides and help for new Uru players.
http://www.cyanworlds.com/   Cyan Worlds Official site
http://www.mystmovie.com  Myst feature film project

Reviews of an earlier version of Uru:
Myst Online: Uru Live <-GameRankings.com.  Aggregate of all Uru reviews was 82.67/100, at a time when the game cost $10/month to play, or $6.95/month with Gametap subscription. 
Now it costs NOTHING, which makes it an incredible value and one of the best free games anywhere on the internet.

Note: This is a non-profit fansite, and not an official site endorsed or sanctioned by Cyan Worlds.