LEGO Universe isn't going wholly free to play, but starting now, players can go through a good portion of the MMO without having to commit to a monthly fee. Access to two adventure zones, one player property area in which you can build LEGO models, and other content is some of what you'll get.
In order to experience everything being offered, including future content updates, you must have a full membership; these are $10 per month. You can see how LEGO Universe is divided between free and premium players on this page.
I know there are a lot of free-to-play titles fighting for your time these days, but this really is a fun one.
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