The short answer is no. You are not legally obligated to use a real estate agent when shopping for a home. However, when you find a home that you want to buy, it may be represented by a real estate agent, rather than For Sale By Owner.
Originally, the Multiple Listing Service (MLS) was only accessible to licensed real estate agents and brokers, as a central network to list homes for sale. Zillow® broke the barrier on that and made MLS listings available to everyone. Shortly thereafter, more Internet based companies like Redfin® and Trulia® entered the digital market. All use the MLS as the data source for displaying homes for sale.
Ninety-three percent of home buyers are now using the Internet to find their next home online. They don’t need an agent to help them find a home, but generally speaking it is recommended that buyers have the benefit of being represented by an agent to handle negotiations, legal paperwork, contract execution, inspections, escrow and title.