While there are a few websites dedicated to "Open Houses", none have created mass at this time, and their compilation of open houses is very incomplete. I am not familiar w/ a search option on Trulia to bring up a list of open houses, but a few listings do note when there is an open house.
You can compile your own list from a few sources and gather a fairly complete list. Check Realtor.com (not all Realtors go on Realtor.com and ck off Open Houses - they cannot unless they have paid subscription fees). Check the online and print editions of your local newspaper, particularly the classifieds. The MLS that services your area may make this available to consumers on a public site, or a local Realtor may be able to provide open houses noted on the MLS system.