There are few sounds sweeter than rain on the roof of a cozy home — but one of them is hearing that your home is worth lots more than you thought.

Knowing your home’s value is critical when you’re consideringequity loans, questioning tax assessments, and, of course, buying or selling. And today that information is much easier to get, thanks to online home-valuation tools that let anybody plug in an address and instantly see a dollar amount.

Home-value websites have exploded in popularity since Seattle-based Zillow.com went online in early 2006. By the end of the year, five million people a month were clicking on the entertaining and easy-to-use site. Zillow (and most other sites) runs public data on more than 60 million homes and 20 million sales through computer algorithms to estimate sale prices. Online valuations aren’t quite as accurate as those made by human appraisers, but they are fun and offer a starting point for real estate discussions. Some are free (supported by advertising), others charge a fee, and a few make you consult with a real estate agent to get your free estimate. Here are five popular sites and what makes each different.

By Mark Henricks

Zillow.com
Charges no fees and requires no personal data. Type in any street address and zip code to get a “Zestimate” of the home’s possible sale price. Fun features include a City Heat Map, which displays a city’s most expensive areas in color codes.

RealEstateABC.com
Works similarly to Zillow but offers more comparable sales and has a simple sliding tool so you can adjust values based on market conditions; the service is free.

HouseValues.com
Makes money by billing real estate agents for referrals. The site feeds data you supply to a local agent, who will contact you with a no-charge market analysis.

ElectronicAppraiser.com
Supplies an elaborate estimate, including maps, sales history, and valuation trends. But it’ll cost you $30 a pop.

InstantHomeValue.com
Charges $99 for an in-depth computer-generated report that’s based on information you give to the site.