There are a few reasons to be interested in finding out the value of your home; refinancing your house, putting it on the market to sell, purchasing replacement value home insurance or just plain curiousity.

You can get an estimate of your home's value using an online service such as Zillow.com. While this may not be exact, using this service can help you get a ballpark figure.
There are a number of situations in which you may need to know the value of your home:
Alternatively, you may just want to know how much your house is worth for curiosities sake.
If you are refinancing your home, the bank will want to ensure that your home is sufficiently valuable to act as collateral for the entire amount they are lending you. As such, the bank will usually require a formal appraisal be done on your home before they give you a loan. You will normally be responsible for paying the cost of this appraisal, which can be anywhere from $100 to $400 or more depending on the size of your home.
A professional appraisal is one of the most exact ways to find out how much your home is worth. The appraiser will compare your home and its features to other homes that have sold in your local area and, using statistical data and information, will make a professional guess or estimate as to your home's value.
If you are purchasing replacement value home insurance, as opposed to market value home insurance, you need to determine how much it would cost to build your actual house again. This number can be very different than the market value- which is the amount your home would sell for- and can even be different than the appraised value. The discrepancy comes when property values have fallen, or when you have installed unique items that do not add a lot of equity to the home, such as extremely fancy showers or bathroom fixtures or anything else that wouldn't add a lot to the actual resale value of a home.
To determine your home's replacement value, you can consult with your insurance agent. Many insurance companies also include forms to help you figure out replacement value. Speaking to a builder to find out the cost per square foot to build a house similar to yours, and also asking about the cost to replace any special features, can also help you determine the appropriate replacement value to use when you are insuring your home.
Selling your home really allows you to find out its worth, since a home is worth only as much as someone pays for it. Your real estate agent will normally price your home for you, based on his experience and on other recent home sales in your area. If the home does not sell for the listed price, you may need to drop the price until you find an interested buyer or you may need to accept a low-ball offer if you have to move quickly.