I have never seen an online up to date compilation of upcoming open houses. In my experience, the best thing to do is look in the newspaper, on Craigslist, and ultimately go drive around the neighborhoods you are interested in on a sunday afternoon and look for the little flags. In any event, the online sites like realtor.com, ziprealty (very good, with days on market info), prudential and coldwell, are all kept pretty up to date with what is in the MLS. You can narrow your search in about 2 seconds using them, and then call the listing agent to see a particular house (might as well bother them since they will definitely get paid when (if) the thing sells).
