Zillow - the US's premier property portal - has added an interesting new innovation... a kind of specialist Q&A feature along the lines of Yahoo Answers. The idea is that you can type in the address of a property, then post a question, or view questions and answer them (if, say, you're the agent, or the owner, or just somebody who's viewed the property). Very nice.
BrightSale have had this for a while now, you can ask a question directly to a seller of a property, the agent is cc'd. Conversations are private to your own email though rather than posted to the site.