The Page Rank service from Google is the most popular way to measure the quality and popularity of a site but it alone isn't enough to give us the correct idea if a site is good or not. There are too many sites with inflated PR, so we can't trust PR alone.

The principle of PR is that sites are divided into 11 categories with ranks from 0 to 10, respectively (plus the N/A category of sites that are not included in the ranking as well). The concept is that the higher the PR, the better the site. Sites that have a PR of 10 are very rare. Sites with PR of 7-9 are more common but still they are a minority because this page rank denotes a very high quality site. If a site has a PR of 5 or 6, this means this site is viewed by Google as a quality site. PR of 3 and 4 are for sites that are about the average. PR of 0 to 2 are for sites that are below the average and therefore aren't the top back linking candidate.

Unlike PR, Alexa doesn't divide sites in groups. Rather, it arranges them in a list. The most popular sites, such as Google, Facebook, or Twitter are at the top. The concept here is that the lower the Alexa ranking, the more popular the site is. Sites with Alexa ranking under 10K are considered exceptionally popular. Sites with Alexa ranking of 10K to 30K enjoy very good popularity. Sites of Alexa ranking up to 100K do get more than decent traffic. It could be speculated that good sites span up to 200-250K and everything above this are sites with low traffic.

It is best if you use Alexa together with PR. While there is some correlation between the two, it is not an exception to find sites with PR6 and Alexa of more than 100K, or with PR3 or 4 and Alexa over a million. Just cross-check a site in two or more ranking services and this will allow you to determine its relevance. One of the perks of Alexa is that it also allows to see which country the majority of a site's traffic comes from.

Compete.com isn't as through as the first two services because they gather their data from a sample of 2 million US users only but still it helps to get an idea if a site is popular in the US or not. When you analyze Compete data, you will notice that frequently sites with good PR and Alexa are almost unheard of in the States and vice versa.

Quantcast is also a service targeted mainly at the US market. It also gathers data from a sample rather than the general Internet population but in its case the sample is comprised of Web site masters, who insert a code snipped on their sites, thus allowing to track visitors. Quantcast also gather their data from ISPs and ad companies. Needless to say, Quantcast isn't very reliable because there are millions of great sites that don't use its services but still it is one more service to check, if you are wondering if a site is good or not.

CustomRank.com provides a service that combines several metrics at once to offer a joint ranking. At present, the services it aggregates are MozTrust, MozRank, PageAuthority, DomainAuthority, Alexa, as well as some other metrics, such as load times, date created, and whether it is listed in DMOZ or not. The good thing about CustomRank is that it tries to balance one service with the other to produce a fair rating.

MozTrust, MozRank, together with DomainAuthority and PageAuthority are all methodologies developed by SEOmoz, one of the most reputable names in SEO. Therefore, it can be concluded that these two metrics are very useful for SEO purposes. MozTrust measures the global link trust score, while MozRank measures link popularity. The more reputable a site's backlinks are, the higher the MozTrust score. Both MozTrust and MozRank use scales from 1 to 10 and here the rule is the higher, the better.

ComScore is another company that uses a sample of 2 million users to provide rankings and therefore isn't as reliable as PR or Alexa. Similarly to Compete and Quantcast, it is more useful when you plan your advertising budget than when you are looking for sites to get backlinks from but still the top sites on the Internet are present in ComScore's lists, so this service can be used as well.

Google Trends is mainly about search volume of keywords but one of its less known uses is to compare how two sites fare over time or in different regions. So, if you have some sites you want to get past data for, so that you can predict their possible future popularity, you can do it with Google Trends (Alexa also offers the trendline for a site over time).

Ranking.com is one more service to consider if you are dissatisfied with the rest. It's kind of limited because only the top one million websites are included but if the site you are interested in is not on the list, this speaks a lot about its popularity. Ranking collects its data from an IE browser toolbar, which means the data is skewed because no Firefox and Apple users are part of the sample.

The list of ranking services doesn't end here but these are the ones that are more or less the top ones. Use them in conjunction (i.e. 3 or more together) to determine if a site is good for backlinks. Some of these services are available as Firefox extensions, so get the extension and you will know more about a site the moment you open it.