We shouldn't debate points regarding Trulia's lack of IDX feeds or claims that Long and Foster and Redfin (which are affiliated with one another) and HomesDataBase offer better search tools. I will emphasize, however, that Trulia offers an outstanding Q&A section -- I think much more user friendly than other real estate-oriented websites. Very obviously, a buyer does not need an agent to see homes online, but even the other recommended websites only offer a portion of all homes available for sale. For example, some agents and firms, such as TTR Sotheby's International Realty, maintain lists of "private exclusive" property listings -- homes which are available for sale but not publicly advertised on any of these websites. Further, some agents and firms also have access to larger listings of foreclosure and REO (bank-owned) properties, most of which are not on these websites.
In short, if you are actively looking to buy a home, you are best served by an agent who understands that a property search is about more than which online search engine is the best.