Myst Online: Uru Live is back! Again! Cyan Worlds has opened a new server and made the game free-to-play, so there's nothing holding you back from making an account. Well, except for the fact that the account creation page is a bit slow right now due to to the overwhelming traffic that is hammering the site.
Cyan is referring to the project as MO:ULagain, and they will gladly accept PayPal donations to offset the server and staff costs. You're not required to pay, but a $5 or $10 donation would certainly help support the game. The company ultimately plans to move Myst Online to open source.