Here is the question I get asked all the time. When someone comes into Rainbow Funding to sell their house, this is the main question on their mind. What agents do is look at current trends in the market and what other houses have sold for that are comparable and then make the most educated guess.
Never let someone sway you into selling your house with them just because it sounds like you will get more money. I could tell someone that I would list their house in Newark at $2 million but that doesn't mean it is worth that price, or anyone will buy it. Your best bet is to do some research on your own then go with the agent or company you are most comfortable with.
If selling your house is an option for you or you just want to know what your house is worth, below is a great link with graphs of the current trends. Check it out.