If you are listed with an agent, let them do it - there are a number of sites to list open houses on.
If you are not listed, you should google around and see how other people are getting their open houses noticed.
I am not aware of any means of designating a property for an open house on Trulia unless they have added that feature very recently.
You can shoot me the date/time and details on the house and I'll take a quick look if it's relevant to any of my current clients.
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