From the Guild Wars website:

ArenaNet and NCsoft are proud to announce the second Guild Wars World Preview Event! During the last three days of October, players from around the world are invited to play Guild Wars and enjoy a completely new Guild Wars experience …

… link also has details about the three ways to try out Guild Wars during the event. I tried this game during the first preview; I enjoyed it. I’m not fanboisterous about it, but their design of instancing and character levels/skills is at least different.