Check out some of the national search sites:

Trulia
A great place to start your home search! Most of the major real estate companies in the area share their listings here. The only problem is the site might not be completely up to date, and tends to have a number of homes listed for sale that have already sold.

Zillow
This site is great for giving you a rough approximation or your home's value by comparing it to prices of nearby homes. Free and easy to use, but keep in mind these are only ROUGH estimations, and don't take into account any special features or the condition of your home.

Cyberhomes
Another site that is great for getting a rough estimate of the value of your home. You can also search homes for sale here. Only houses with an orange "for sale" banner on them in the results pages are actually for sale. Slightly slower load times than Zillow or Trulia.

Realtor.com
One of the largest databases of homes for sale in the nation. However, the content is not always current and homes sometime linger for weeks after being sold, so there's no guarantee the home you are looking at is still for sale. Also, they charge large fees for hosting virtual tours, and other agent content, so many agents opt out of putting all of their information here. However, very easy to use and load times are fast.

The Real Estate Book
A decent national database of listings. The search feature has less options for filtering results, but it does contain a large amount of listings.

Vast
VAST is another national site that provides some limited information, but we suggest you make it the last of the above sites you go to for your home search. It does contain a good number of homes, but we haven't found much useful, original information here. Still, they appear to be newer and growing, so perhaps their services will improve with time.

City-info
City-info gives you detailed census information based on zipcode or city. For cities with populations above 6,000 we prefer pulling data by state and then city to get more accurate information about what is (usually) a smaller area. Great if you want to know the average age of the area you're looking at, crime statistics, information on local schools, and much, much more.