As a web designer or developer building a site, it’s helpful to first scout similar websites (i.e. the competition) to help you make design and development decisions. Knowing what websites work and which ones tank can give you insights and inspiration on what design elements, website features, and website content is effective.

To help you in your analysis, here are 15 top-notch, free, and simple web tools to help you learn more about a particular website.

1. Yahoo! Site Explorer

Site explorer allows you to inspect certain aspects of a URL. The “Inlinks” feature shows you details about web pages that link back to the particular URL you are exploring. Site Explorer gives you some insights on how popular a website is based on how many web pages are linking to it.

2. Compete

Compete is a web-based analytics tools that allows you to compare the traffic statistics and trends of up to three websites. It gives you an estimate of how many unique visitors and page impressions a website gets and how fast a website is growing in popularity (called Growth Velocity).

3. Quantcast

Quantcast is another popular site traffic measuring tool. Quantcast allows publishers to “Quantify” their website - a process in which you place a script in your web pages so that they can measure your traffic statistics directly. With that said - sites that aren’t quantified may not have accurate/complete data and many people aren’t comfortable about placing third-party scripts on their websites.

4. Alexa

Started in 1996, Alexa is the most popular traffic ranking service today. It tracks traffic statistics of websites and provides a numerical rank based on the data that they collect. You can find a website’s Page Views and Reach (the % of internet users that have visited the website).

5. Feed Compare

An effective way to determine a website’s popularity is by tracking its RSS feed subscriber trends. If a website uses FeedBurner - the leading provider of RSS feeds services - you can use Feed Compare to see trends in subscribership from websites and compare growth/decline of subscribers for up to 4 feeds at one time.

6. popuri.us

popuri.us is a tool that can help determine how popular a URL is based on web services and statistics such as Alexa, Technorati, Delicious bookmarks, number of subscribers, and more. It’s a one page tool that gives you a quick overview of how popular a website is. popuri.us even gives you the option to post a widget on your site to display your popularity.

7. socialmeter

socialmeter is a web tool that scans how popular a web page is throughout major social websites such as Digg, Stumbleupon, and Delicious. It gives you a “socialmeter score” which is currently just a summation of the results found (useful for comparing social media website popularity of different websites).

8. TweetVolume

Another way of monitoring a website’s popularity is to see if people on Twitter are talking about it. TweetVolume allows you to search key words and phrases (i.e. - a website’s name, URL, author name, etc.) to see how many tweets have mentioned your search terms.

9. Quarkbase

Quarkbase is a comprehensive web tool that presents a myriad of information about a website. Quarkbase has a “Social Popularity” tab that shows you information about a website’s popularity among social media sites like Digg, Stumbleupon, and Delicious.

10. BlogPulse Profiles

If you’re interested in finding data about a specific blog or blogger, The BlogPulse Profiles tool is a simple web application that provides a blog’s rank, how often it’s cited in other blogs, and other information such as how many posts are published each month and blogs that are similar to it.

11. Technorati Blogging Central

Technorati is the leading blog search engine. Technorati’s Blogger Central allows you to see the ranks of a particular blog to gauge its popularity among the 128 million+ blogs in the world. You can find the Top 100 blogs in Blogger Central. Technorati determines a blog’s rank by the number of reactions (links that go to the blog) it has.

12. Statbrain.com

Statbrain.com is a simple tool that tells you how many visitors a website receives per day.

13. Cubestat

Cubestat is another simple web tool that provides you with a host of information about a website’s popularity daily page views. What’s unique about Cubestat is that it estimates a website’s monetary value (in U.S. dollars) - the more expensive the website is, the more popular it is.

14. dnScoop

dnScoop attempts to estimate the value of a website based on factors such as links pointing to the domain, popularity of the domain, page rank, traffic, and more.

15. WebsiteOutlook

WebsiteOutlook allows you to determine a website’s popularity based on its estimated value, daily page views, and revenue.

What’s your method of measurement?

How do you measure a website’s popularity? Are there tools and web services that should be on this list? Share it with us in the comments!