MyBB Community Forums

Okay, the title is a bit confusing, but it best explains this.
How did they post like this, without it having a user?
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Uh... I'm not really sure what you mean. Are you asking how to allow people to post without a user account?

Please don't link to hacking sites.

That's not a post, it's a page separate from the forum. You can do something similar with this plugin - http://community.mybb.com/thread-63082.html