A BPO is a Broker Price Opinion. It is basically a Home Valuation - but not as detailed as an appraisal. A lot of banks will ask real estate agents to do BPO's so they have a "basic" valuation of a home to determine it's value in a short sale, or pending foreclosure situation. The banks usually pay a BPO Manager a fee of maybe $150 and then that company hires the local agent and pays them $50 - $75 to fill out their form. Many agents do this in hopes that they will get the listing in the future, but usually that will not happen and they are only doing it to make a few bucks. About 85% of the BPO's do not require that you go into the house. They only want the agent to drive by and make sure the house is in fact there, and not a total disaster.
If you are just looking for an opinion of value for your home I would call a real estate agent. Most agents will do a CMA for free in hopes of making a connection with you and gaining your business. In, NC, real estate agents are allowed to do CMA's (Comparative Market Analysis) but not BPO's, as they are competing with the Appraisers. So you should just ask for a CMA.
If you are looking for a BPO because you may have a short sale situation on your hands, your bank will order it. They will only accept them from certain vendors on their form and they would not ask you for one.
If you look at Zillow, you can get a pretty good Market Condition Trend graph for your area or an address.