Sometimes the best way to improve your own business is by taking a hard look at your competitors. By understanding their most and least effective strategies, you can fine-tune your own marketing efforts.

Here is a list of competitive research tools. There are tools to research your competition’s keywords, linking strategies, traffic metrics, and more. There are both free and paid tools. If you have any other favorite resources to research your competitors, be sure to add them in the comments below.

Free Tools

Alexa. Search Alexa by keyword or category for traffic trends on competing sites. Get rank, related keywords, and site analytics for free.

Google AdWords Keyword Tool. Plug in a URL and get the keyword ideas Google thinks the page should be advertising for, as well as the monthly keyword volume estimates.

Google Trends for Websites. This Google tool gives you traffic estimates, related websites, and related keywords. Compare trends between multiple sites. Data is only available for higher traffic sites.

Archive.org. This site provides free access on how old a website is and what it looked like in the past.

Google Traffic Estimator. Enter a website and get estimates on the number of AdWords ad clicks and bid prices for the top ad position.

SearchStatus. SearchStatus lets you view a site’s Google PageRank, Google Category, Alexa popularity ranking, Compete.com ranking, SEOmoz Linkscape mozRank, Alexa related links and backward links.

DomainTools. Search "Whois" domain ownership data. Or get reverse Whois information, if you know domain owners and want to know which sites they own.