Finding comps (comparative data, where we determine the value of your home based on the sales prices of homes like and near yours) is as simple as entering information into an online form. Keep in mind, though, that your real estate sales professional will almost always give you more accurate information, so use these as baselines or starting points.

Finding Real Estate Comps (Comparable Real Estate Sales)

Free Home Value Websites

Warning - Free sites typically provide automated estimates from their database using pre-determined rules.  This may lead to inaccurate results when properties of significantly different values are close together geographically.  Use these as a guide rather than as solid comparables.
 
Cyberhomes
From Fidelity National Financial, this site is information rich. Find comps, get charts, maps, and graphs. This one stands out most with its heat maps and buy/sell indicator charts. You can also refine the value that is determined by adding improvements, correcting square footage, and more.  Best of the free sites.

Zillow.com
Find the Zestimate of any property's worth using comparable data. Zillow mashes this data up with Google maps so you can see what other properties in the area are worth as well. Includes property info, charts of property worth over time, and value ranges. The site lets you refine a property's value, like Cyberhomes, by adding improvements and changing other details about the property. You can also use the site to list a property for sale.  Note Zillow's data sources & methodology cause wide variation in accuracy by state and county, especially in areas with lower transaction volume.  Use with caution!

Pay Per Property Report Sites

ElectronicAppraiser.com
- RECOMMENDED -
This is the best of the pay per report sites.  Their underlying database is the same used by mortgage loan underwriters for their own desktop underwriting.  

Domania.com
They will send you the market value of your home as determined by a local realtor for free -- and they'll call you to solicit your business.  Automated valuation reports are charged at various rates depending on the amount of data requested.

Eppraisal
Eppraisal looks at comps in your area and determines the price value range for your home. It's a fairly reliable site with useful features, although it's not quite as comprehensive as some others.

Realtor.com
Basically, this site now allows you to determine the value of your home by searching for homes that recently sold in your area. It's essentially a stripped down version of what your real estate agent will use. What you're also missing is the knowledge and experience of your local real estate agent (very important).

Home Smart Reports
You can get property information, nearby sales with map, and a neighborhood summary at no cost. For a fee, you can get a much more advanced report with risk analysis, many more comps, and other statistical charts.

Property Shark
Property Shark provides public data on a property, recent sales, sale history, and comparables. The good news is that you don't have to provide your contact information to this site.

Property Value Subscription Services

RealQuest.com
Retrieve nearly everything you'd need to know for home values. Basics include: Property Detail Report, Comparable Sales, Parcel Map/Assessor Map, Street Map, Neighborhood Information, Legal and Vesting, Automated Valuation (AVM), Transaction History, Custom Searches, and Flood Maps. Other reports include: statement of liens, foreclosure activity, fraud checks, transaction histories, and more.

DataQuick.com
Similar to RealQuest.com.  Shop both to see which better fits your investing needs.